Intersil ISL6251AHAZ, ISL6251AHRZ, ISL6251HAZ, ISL6251HRZ Schematics

Low Cost Multi-Chemistry Battery Charger Controller

ISL6251, ISL6251A

The ISL6251, ISL6251A is a highly integrated battery charger
controller for Li-Ion/Li-Ion polymer batteries and NiMH
batteries. High efficiency is achieved by a synchronous buck
topology and the use of a MOSFET, instead of a diode, for
selecting power from the adapter or battery. The low side
MOSFET emulates a diode at light loads to improve the light
load efficiency and prevent system bus boosting.

The constant output voltage can be selected for 2, 3 and 4
series Li-Ion cells with 0.5% accuracy over-temperature. It can
be also programmed between 4.2V+5%/cell and 4.2V-5%/cell
to optimize battery capacity. When supplying the load and
battery charger simultaneously, the input current limit for the
AC adapter is programmable to within 3% accuracy to avoid
overloading the AC adapter, and to allow the system to make
efficient use of available adapter power for charging. It also
has a wide range of programmable charging current. The
ISL6251, ISL6251A provides outputs that are used to monitor
the current drawn from the AC adapter, and monitor for the
presence of an AC adapter. The ISL6251, ISL6251A
automatically transitions from regulating current mode to
regulating voltage mode.

Ordering Information

TEMP

PART NUMBER

(Notes 1, 2, 3)

ISL6251HRZ ISL 6251HRZ -10 to +100 28 Ld 5x5 QFN L28.5×5

ISL6251HAZ ISL 6251HAZ -10 to +100 24 Ld QSOP M24.15

ISL6251AHRZ ISL6251 AHRZ -10 to +100 28 Ld 5x5 QFN L28.5×5

ISL6251AHAZ ISL6251 AHAZ -10 to +100 24 Ld QSOP M24.15

NOTES:

1. Add “-T*” suffix for tape and reel. Please refer to TB347
reel specifications.

2. These Intersil Pb-free plastic packaged products employ special Pb­free material sets, molding compounds/die attach materials, and
100% matte tin plate plus anneal (e3 termination finish, which is
RoHS compliant and compatible with both SnPb and Pb-free
soldering operations). Intersil Pb-free products are MSL classified at
Pb-free peak reflow temperatures that meet or exceed the Pb-free
requirements of IPC/JEDEC J STD-020.

3. For Moisture Sensitivity Level (MSL), please see device information
page for ISL6251
see techbrief TB363

PART

MARKING

, ISL6251A. For more information on MSL please

.

RANGE

(°C)

PACKAGE

(Pb-Free)

PKG.

DWG. #

for details on

Features

• ±0.5% Charge Voltage Accuracy (-10°C to +100°C)

• ±3% Accurate Input Current Limit

• ±3% Accurate Battery Charge Current Limit

• ±25% Accurate Battery Trickle Charge Current Limit
(ISL6251A)

• Programmable Charge Current Limit, Adapter Current Limit
and Charge Voltage

• Fixed 300kHz PWM Synchronous Buck Controller with Diode
Emulation at Light Load

• Output for Current Drawn from AC Adapter

• AC Adapter Present Indicator

• Fast Input Current Limit Response

• Input Voltage Range 7V to 25V

• Supports 2, 3 and 4 Cells Battery Pack

• Up to 17.64V Battery-Voltage Set Point

•Thermal Shutdown

• Support Pulse Charging

• Less than 10µA Battery Leakage Current

• Charge Any Battery Chemistry: Li-Ion, NiCd, NiMH, etc.

• Pb-Free (RoHS Compliant)

Applications

• Notebook, Desknote and Sub-notebook Computers

• Personal Digital Assistant

March 13, 2014
FN9202.3

1

CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.

1-888-INTERSIL or 1-888-468-3774

Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries.

|Copyright Intersil Americas LLC 2005-2006, 2014. All Rights Reserved

All other trademarks mentioned are the property of their respective owners.

Pin Configurations

1

2

3

4

5

6

7

21

20

19

18

17

16

15

28 27 26 25 24 23 22

8 9 10 11 12 13 14

EN

CELLS

ICOMP

VCOMP

ICM

VREF

CHLIM

ACLIM

PGND

LGATE

VDDP

BOOT

CSOP

PHASE

UGATE

ACSET

DCIN

ACPRN

CSON

VADJ

GND

CSIN

CSIP

NA

NA

NA

VDD

NA

DCIN

124

VDD

223

ACPRN ACSET

322

CSON

421

CSOP

EN

520

CSIN

CELLS

619

CSIP

ICOMP

718

VCOMP

817

ICM

916

PHASE

VREF

10 15

UGATE

CHLIM

11 14

BOOT

ACLIM

12 13

VDDP

VADJ LGATE

GND PGND

ISL6251, ISL6251A

(28 LD QFN)

TOP VIEW

ISL6251, ISL6251A

ISL6251, ISL6251A

(24 LD QSOP)

TOP VIEW

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ISL6251, ISL6251A

Absolute Maximum Ratings Thermal Information

DCIN, CSIP, CSON to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +28V

CSIP-CSIN, CSOP-CSON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +0.3V

PHASE to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -7V to 30V

BOOT to PGND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +35V

ACLIM, ACPRN, CHLIM, VDD to GND . . . . . . . . . . . . . . . . . . . . . . -0.3V to 7V

BOOT-PHASE, VDDP-PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 7V

ICM, ICOMP, VCOMP to GND . . . . . . . . . . . . . . . . . . . . . . .-0.3V to VDD+0.3V

VREF, CELLS to GND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-0.3V to VDD+0.3V

EN, VADJ, PGND to GND . . . . . . . . . . . . . . . . . . . . . . . . . .-0.3V to VDD+0.3V

UGATE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PHASE-0.3V to BOOT+0.3V

LGATE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PGND-0.3V to VDDP+0.3V

CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product
reliability and result in failures not covered by warranty.

NOTES:

4. When the voltage across ACSET is below 0V, the current through ACSET should be limited to less than 1mA.

is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See Tech

5. θ

JA

Brief TB379

is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.

6. θ

JA

7. Fo r θ

8. BOOT-PHASE voltage is -0.3V to -0.7V during start-up. This is due to a small current (<1mA) that flows from the battery to the PHASE pin and to an
internal current sink on the BOOT pin through an internal diode. This does not harm the part.

.

, the “case temp” location is the center of the exposed metal pad on the package underside.

JC

Thermal Resistance θ

QFN Package (Notes 5, 7) . . . . . . . . . . . . . . 39 9.5

QSOP Package (Note 6). . . . . . . . . . . . . . . . 88 N/A

Junction Temperature Range . . . . . . . . . . . . . . . . . . . . . . .-10°C to +150°C

Operating Temperature Range . . . . . . . . . . . . . . . . . . . . . .-10°C to +100°C

Storage Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-65°C to +150°C

Pb-Free Reflow Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . see link below

http://www.intersil.com/pbfree/Pb-FreeReflow.asp

(°C/W) θJC (°C/W)

JA

Electrical Specifications DCIN = CSIP = CSIN = 18V, CSOP = CSON = 12V, ACSET = 1.5V, ACLIM = VREF, VADJ = Floating, EN = VDD = 5V,

BOOT-PHASE = 5.0V, GND = PGND = 0V, C

the operating temperature range, -10°C to +100°C.

PARAMETER TEST CONDITIONS

SUPPLY AND BIAS REGULATOR

DCIN Input Voltage Range 725V

DCIN Quiescent Current EN = VDD or GND, 7V ≤ DCIN ≤ 25V 1.4 3 mA

Battery Leakage Current (Note 9) DCIN = 0, no load 3 10 µA

VDD Output Voltage/Regulation 7V ≤ DCIN ≤ 25V, 0 ≤ I

VDD Undervoltage Lockout Trip Point VDD Rising 4.0 4.4 4.6 V

Reference Output Voltage VREF 0 ≤ I

Battery Charge Voltage Accuracy CSON = 16.8V, CELLS = VDD, VADJ = Float -0.5 0.5 %

TRIP POINTS

ACSET Threshold 1.24 1.26 1.28 V

ACSET Input Bias Current Hysteresis 2.2 3.4 4.4 µA

ACSET Input Bias Current ACSET ≥ 1.26V 2.2 3.4 4.4 µA

= 1µF, I

VDD

Hysteresis 200 250 400 mV

VREF

CSON = 12.6V, CELLS = GND, VADJ = Float -0.5 0.5 %

CSON = 8.4V, CELLS = Float, VADJ = Float -0.5 0.5 %

CSON = 17.64V, CELLS = VDD, VADJ = VREF -0.5 0.5 %

CSON = 13.23V, CELLS = GND, VADJ = VREF -0.5 0.5 %

CSON = 8.82V, CELLS = Float, VADJ = VREF -0.5 0.5 %

CSON = 15.96V, CELLS = VDD, VADJ = GND -0.5 0.5 %

CSON = 11.97V, CELLS = GND, VADJ = GND -0.5 0.5 %

CSON = 7.98V, CELLS = Float, VADJ = GND -0.5 0.5 %

= 0mA, TA = -10°C to +100°C, TJ≤ 125°C, unless otherwise noted. Boldface limits apply over

VDD

MIN

(Note 10) TYP

≤ 30mA 4.925 5.075 5.225 V

VDD

≤ 300µA 2.365 2.39 2.415 V

MAX

(Note 10) UNITS

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ISL6251, ISL6251A

Electrical Specifications DCIN = CSIP = CSIN = 18V, CSOP = CSON = 12V, ACSET = 1.5V, ACLIM = VREF, VADJ = Floating, EN = VDD = 5V,

BOOT-PHASE = 5.0V, GND = PGND = 0V, C

the operating temperature range, -10°C to +100°C.

PARAMETER TEST CONDITIONS

ACSET Input Bias Current ACSET < 1.26V -1 0 1 µA

OSCILLATOR

Frequency 245 300 355 kHz

PWM Ramp Voltage (Peak-Peak) CSIP = 18V 1.6 V

SYNCHRONOUS BUCK REGULATOR

Maximum Duty Cycle 97 99 99.6 %

UGATE Pull-Up Resistance BOOT-PHASE = 5V, 500mA source current 1.8 3.0 Ω

UGATE Source Current BOOT-PHASE = 5V, BOOT-UGATE = 2.5V 1.0 A

UGATE Pull-Down Resistance BOOT-PHASE = 5V, 500mA sink current 1.0 1.8 Ω

UGATE Sink Current BOOT-PHASE = 5V, UGATE-PHASE = 2.5V 1.8 A

LGATE Pull-Up Resistance VDDP-PGND = 5V, 500mA source current 1.8 3.0 Ω

LGATE Source Current VDDP-PGND = 5V, VDDP-LGATE = 2.5V 1.0 A

LGATE Pull-DOWN Resistance VDDP-PGND = 5V, 500mA sink current 1.0 1.8 Ω

LGATE Sink Current VDDP-PGND = 5V, LGATE = 2.5V 1.8 A

CHARGING CURRENT SENSING AMPLIFIER

Input Common-Mode Range 018V

Input Offset Voltage Note 11 -2.5 0 2.5 mV

Input Bias Current at CSOP 0 < CSOP < 18V 0.25 2 µA

Input Bias Current at CSON 0 < CSON < 18V 75 100 µA

CHLIM Input Voltage Range 03.6V

CSOP to CSON Full-Scale Current Sense
Voltage

CHLIM Input Bias Current CHLIM = GND or 3.3V, DCIN = 0V -1 1 µA

CHLIM Power-Down Mode Threshold
Voltage

CHLIM Power-Down Mode Hysteresis
Voltage

ADAPTER CURRENT SENSING AMPLIFIER

Input Common-Mode Range

Input Offset Voltage Note 11 -2 2 mV

Input Bias Current at CSIP and CSIN
Combined

Input Bias Current at CSIN 0 < CSIN < DCIN (Note 11) 0.10 1 µA

= 1µF, I

VDD

CSIP = 11V 1 V

ISL6251: CHLIM = 3.3V 157 165 173 mV

ISL6251A, CHLIM = 3.3V 160 165 170 mV

ISL6251: CHLIM = 2.0V 95 100 105 mV

ISL6251A: CHLIM = 2.0V 97 100 103 mV

ISL6251: CHLIM = 0.2V 5.0 10 15.0 mV

ISL6251A: CHLIM = 0.2V 7. 5 10 12.5 mV

CHLIM rising 80 88 95 mV

CSIP = CSIN = 25V 100 130 µA

= 0mA, TA = -10°C to +100°C, TJ≤ 125°C, unless otherwise noted. Boldface limits apply over

VDD

(Continued)

MIN

(Note 10) TYP

15 25 40 mV

7 25 V

MAX

(Note 10) UNITS

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ISL6251, ISL6251A

Electrical Specifications DCIN = CSIP = CSIN = 18V, CSOP = CSON = 12V, ACSET = 1.5V, ACLIM = VREF, VADJ = Floating, EN = VDD = 5V,

BOOT-PHASE = 5.0V, GND = PGND = 0V, C

the operating temperature range, -10°C to +100°C.

PARAMETER TEST CONDITIONS

ADAPTER CURRENT LIMIT THRESHOLD

CSIP to CSIN Full-Scale Current Sense
Voltage

ACLIM Input Bias Current ACLIM = VREF 10 16 20 µA

VOLTAGE REGULATION ERROR AMPLIFIER

Error Amplifier Transconductance from
CSON to VCOMP

CURRENT REGULATION ERROR AMPLIFIER

Charging Current Error Amplifier
Transconductance

Adapter Current Error Amplifier
Transconductance

BATTERY CELL SELECTOR

CELLS Input Voltage for 4 Cell Select 4.3 V

CELLS Input Voltage for 3 Cell Select 2 V

CELLS Input Voltage for 2 Cell Select 2.1 4.2 V

LOGIC INTERFACE

EN Input Voltage Range 0VDDV

EN Threshold Voltage Rising 1.030 1.06 1.100 V

EN Input Bias Current EN = 2.5V 1.8 2.0 2.2 µA

ACPRN Sink Current ACPRN = 0.4V 3 8 11 mA

ACPRN Leakage Current ACPRN = 5V -0.5 0.5 µA

ICM Output Accuracy
(Vicm = 19.9 x (Vcsip-Vcsin))

Thermal Shutdown Temperature 150 °C

Thermal Shutdown Temperature
Hysteresis

NOTES:

9. This is the sum of currents in these pins (CSIP, CSIN, BOOT, UGATE, PHASE, CSOP, CSON) all tied to 16.8V. No current in pins EN, ACSET, VADJ, CELLS,
ACLIM, CHLIM.

10. Parameters with MIN and/or MAX limits are 100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization
and are not production tested.

11. Limits established by characterization and are not production tested.

= 1µF, I

VDD

ACLIM = VREF 97 100 103 mV

ACLIM = Float 72 75 78 mV

ACLIM = GND 47 50 53 mV

ACLIM = GND -20 -16 -10 µA

CELLS = VDD 30 µA/V

Falling 0.985 1.000 1.025 V

Hysteresis 30 60 90 mV

CSIP - CSIN = 100mV -3 0 +3 %

CSIP - CSIN = 75mV -4 0 +4 %

CSIP - CSIN = 50mV -5 0 +5 %

= 0mA, TA = -10°C to +100°C, TJ≤ 125°C, unless otherwise noted. Boldface limits apply over

VDD

(Continued)

MIN

(Note 10) TYP

50 µA/V

50 µA/V

25 °C

MAX

(Note 10) UNITS

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ISL6251, ISL6251A

-0.6

-0.3

0

0.3

0.6

0 8 16 24 32 40

VDD LOAD REGULATION A CCURACY (% )

VDD=5.075V
EN=0

LOAD CURRENT (mA)

-0.6

-0.3

0

0.3

0.6

0 8 16 24 32 40

VDD LOAD REGULATION A CCURACY (% )

VDD=5.075V
EN=0

LOAD CURRENT (mA)

0

0.02

0.04

0.06

0.08

0.1

0 100 200 300 400

VREF LOAD REGULATION A CCURACY (%)

VREF=2.390V

LOAD CURRENT (µA)

0

0.02

0.04

0.06

0.08

0.1

0 100 200 300 400

VREF LOAD REGULATION A CCURACY (%)

VREF=2.390V

LOAD CURRENT (µA)

CSIP-CSIN (mV)

0

1

2

3

4

5

6

7

8

9

10

10 20 30 40 50 60 70 80 90

100

|

ICM ACCURACY

|

(%)

0.76

0.8

0.84

0.88

0.92

0.96

1

00.511.522.533.54

EFFICIENCY (% )

CHARGE CURRENT (A)

VCSON=16.8V
4 CELLS

VCSON=12.6V

(3 CELLS)

VCSON=8.4V
2 CELLS

0.76

0.8

0.84

0.88

0.92

0.96

1

00.511.522.533.54

EFFICIENCY (% )

CHARGE CURRENT (A)

VCSON=16.8V
4 CELLS

VCSON=12.6V

(3 CELLS)

VCSON=8.4V
2 CELLS

LOAD
CURRENT
5A/div

A

CURRENT
5A/div

CHARGE
CURRENT
2A/div

BATTERY
VOLTAGE
2V/div

LOAD STEP: 0-4A
CHARGE CURRENT: 3A

A

A

CSON
5V/div

EN
5V/div

INDUCTOR
CURRENT
2A/div

CHARGE
CURRENT
2A/div

CSON
5V/div

EN
5V/div

INDUCTOR
CURRENT
2A/div

CHARGE
CURRENT
2A/div

Typical Operating Performance DCIN = 20V, 4S2P Li-Battery, T

FIGURE 1. VDD LOAD REGULATION

FIGURE 2. VREF LOAD REGULATION

= 25°C, unless otherwise noted.

A

FIGURE 3. ICM ACCURACY vs AC ADAPTER CURRENT FIGURE 4. SYSTEM EFFICIENCY vs CHARGE CURRENT

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C ADAPTER CURRENT LIMI T: 5.15

FIGURE 5. LOAD TRANSIENT RESPONSE FIGURE 6. CHARGE ENABLE AND SHUTDOWN

DAPTER

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FN9202.3

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ISL6251, ISL6251A

INDUCTOR
CURRENT
2A/div

CSON
10V/div

VCOMP

ICOMP

BATTERY
INSERTION

BATTERY
REMOVAL

VCOMP
2V/div

ICOMP
2V/div

INDUCTOR
CURRENT
2A/div

CSON
10V/div

VCOMP

ICOMP

BATTERY
INSERTION

BATTERY
REMOVAL

VCOMP
2V/div

ICOMP
2V/div

PHASE
10V/div

INDUCTOR
CURRENT
1A/div

UGATE
5V/div

CHLIM=0.2V
CSON=8V

PHASE
10V/div

INDUCTOR
CURRENT
1A/div

UGATE
5V/div

PHASE
10V/div

INDUCTOR
CURRENT
1A/div

UGATE
5V/div

CHLIM=0.2V
CSON=8V

PHASE
10V/div

LGATE
2V/div

UGATE
2V/div

PHASE
10V/div

LGATE
2V/div

UGATE
2V/div

CHLIM
1V/div

CHARGE
CURRENT
1A/div

Typical Operating Performance DCIN = 20V, 4S2P Li-Battery, T

FIGURE 7. BATTERY INSERTION AND REMOVAL FIGURE 8. SWITCHING WAVEFORMS AT DIODE EMULATION

= 25°C, unless otherwise noted. (Continued)

A

FIGURE 9. SWITCHING WAVEFORMS IN CC MODE FIGURE 10. TRICKLE TO FULL-SCALE CHARGING

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ISL6251, ISL6251A

Functional Pin Descriptions

BOOT

Connect BOOT to a 0.1µF ceramic capacitor to PHASE pin and
connect to the cathode of the bootstrap schottky diode.

UGATE

UGATE is the high side MOSFET gate drive output.

LGATE

LGATE is the low side MOSFET gate drive output; swing between
0V and VDDP.

PHASE

The Phase connection pin connects to the high side MOSFET
source, output inductor, and low side MOSFET drain.

CSOP/CSON

CSOP/CSON is the battery charging current sensing
positive/negative input. The differential voltage across CSOP and
CSON is used to sense the battery charging current, and is
compared with the charging current limit threshold to regulate
the charging current. The CSON pin is also used as the battery
feedback voltage to perform voltage regulation.

CSIP/CSIN

CSIP/CSIN is the AC adapter current sensing positive/negative
input. The differential voltage across CSIP and CSIN is used to
sense the AC adapter current, and is compared with the AC
adapter current limit to regulate the AC adapter current.

GND

GND is an analog ground.

DCIN

The DCIN pin is the input of the internal 5V LDO. Connect it to the
AC adapter output. Connect a 0.1μF ceramic capacitor from DCIN
to PGND.

ACSET

ACSET is an AC adapter detection input. Connect to a resistor
divider from the AC adapter output.

ACPRN

Open-drain output signals AC adapter is present. ACPRN pulls low
when ACSET is higher than 1.26V; and pulled high when ACSET is
lower than 1.26V.

PGND

PGND is the power ground. Connect PGND to the source of the
low side MOSFET for the low side MOSFET gate driver.

VDD

VDD is an internal LDO output to supply IC analog circuit. Connect
a 1μF ceramic capacitor to ground.

VDDP

VDDP is the supply voltage for the low-side MOSFET gate driver.
Connect a 4.7Ω resistor to VDD and a 1μF ceramic capacitor to
power ground.

ICOMP

ICOMP is a current loop error amplifier output.

VCOMP

VCOMP is a voltage loop amplifier output.

CELLS

This pin is used to select the battery voltage. CELLS = VDD for a
4S battery pack, CELLS = GND for a 3S battery pack, CELLS =
Float for a 2S battery pack.

VADJ

VADJ adjusts battery regulation voltage. VADJ = VREF for

4.2V+5%/cell; VADJ = Floating for 4.2V/cell; VADJ = GND for

4.2V-5%/cell. Connect to a resistor divider to program the
desired battery cell voltage between 4.2V-5% and 4.2V+5%.

CHLIM

CHLIM is the battery charge current limit set pin. CHLIM input
voltage range is 0.1V to 3.6V. When CHLIM = 3.3V, the set point
for CSOP-CSON is 165mV. The charger shuts down if CHLIM is
forced below 88mV.

ACLIM

ACLIM is the adapter current limit set pin. ACLIM = VREF for
100mV, ACLIM = Floating for 75mV, and ACLIM = GND for 50mV.
Connect a resistor divider to program the adapter current limit
threshold between 50mV and 100mV.

VREF

VREF is a 2.39V reference output pin. It is internally
compensated. Do not connect a decoupling capacitor.

EN

EN is the Charge Enable input. Connecting EN to high enables the
charge control function, connecting EN to low disables charging
functions. Use with a thermistor to detect a hot battery and
suspend charging.

ICM

ICM is the adapter current output. The output of this pin produces
a voltage proportional to the adapter current.

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FIGURE 11. FUNCTIONAL BLOCK DIAGRAM

T

V

V

-

ge

g

r

P

E

A

t

V

M+-

Min

e

r

V

+

+

-

r

-+-

A

A

CSE

T

A

V

M

-

r

r

E

V

2

M+-PWM+-

V

2

V

-+-

DCIN

×

2

×

0

A

r

t

t

Current

r

V

-

-

-

-

F

K

K

r

e

r

F

K

K

F

K

K

T

V

V

-

ge

g

r

g

r

P

E

A

t

V

M+-PWM+-

Min

e

r

Min

e

r

V

++

+

-

r

r

-+-

A

A

T

A

V

M

-

r

r

E

V

2

M+-PWM+-

V

2

V

-+-

DCIN

×

2

×

0

A

r

t

t

Current

r

Current

r

V

-

-

-

-

F152

k

2

k

r

e

r

F

F

k

k

CSE

CPRN

CPRN

CLIM

CLIM

ICOMP

ICOMP

VCOM P

VCOM P

VADJ

VADJ

VADJ

VADJ

CELLS

CELLS

VRE

VRE

VRE

VRE

152

152

152

152

15

Current Limit Se

Current Limit Se

VRE

VRE

514

514

514

514

ISL6251, ISL6251A

CSIP

IC

CSIP

IC

+

+

CA1

+

+
+

+

CA2

CA2

gm3

gm3
gm3

gm3

CA1

CA1

Current

Current

Current

Buffe

Buffe

Buffe

Buffe

Buffe

Buffe

CA

CA
CA2

CA2

CA

CA

Min

Min

Min

Min

Min

Min

CA1

+

+

2.1

2.1

Volta

Volta
Selector

Selector

+

+

+

+-+

-

-

dapter

dapter

+

+

gm1

gm1

-

-

1.26

1.26

dapte

dapte

1.27

1.27
Curren

Curren

Limit Se

Limit Se

gm1

gm1

2.1

2.1

Voltage

Voltage

Selecto

Selecto

+

+

Min

Min

Voltag

Voltag

Voltag

Voltag

Voltag

Buffe

Buffe

Buffe

Buffe

Buffe

0.25

0.25

0.25

0.25

19.9

19.9

CSIN

CSIN

gm2

gm2

gm2

gm2

LDO

LDO

Regulato

Regulato

Regulato

Regulato

LDO

LDO

LDO

ulato

ulato

ulato

Re

Re

Re

PW

PW

PW

PW

VDDP

VDDP

+

+

VDD

VDD

BOO

BOO

UGATE

UGATE

PHASE

PHASE

VDD

VDD

LGA T

LGA T

LGA T

LGA T

PGND

PGND

1.06

VDD

VDD

VDD

VDD

2

2

CA2

CA2

-

-

CA

CA

+

VREF

VREF

VREF

VREF

Reference

Reference

Reference

Reference

GND

GND

GND

GND

CSON

CSON

+

CSOP

CSOP

CHLIM

CHLIM

CHLIM

CHLIM

1.06

1.065V

1.065V

+

+

+

+

EN

EN

EN

EN

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ISL6251, ISL6251A

FIGURE 12. ISL6251, ISL6251A TYPICAL APPLICATION CIRCUIT WITH FIXED CHARGING PARAMETERS

CSIP

CSIN

BOOT

UGATE

PHASE

LGATE

PGND

CSOP

CSON

CELLS

ICM

GND

DCIN

ACSET

VDDP

VDD

ACPRN

ICOMP

VCOMP

VADJ

EN

CHLIM

ACLIM

VREF

Q1

Q2

L
10µH

C1

10µF

SYSTEM LOAD

C10
10µF

R1
40mΩ

AC ADAPTER

R2
20mΩ

R3

18Ω

R4

2.2Ω

R6:10k

R5
100k

R8

130k

1%

R9

10.2k
1%

C2

0.1µF

C4

0.1µF

C3
1µF

C5:10nF

C6:6.8nF

C9
1µF

ISL6251

ISL6251A

FLOATING

4.2V/CELL

R10

4.7Ω

C7
1µF

3.3V

Battery

Pack

BAT+

BAT-

VDD
4 CELLS

D1

Optional

D2

VDDP

VREF

R12

20k 1%

R11

130k

1%

R13

1.87k
1%

2.6A CHARGE LIMIT
253mA Trickle Charge

C11
3300pF

Q3

Trickle Charge

D4

To Host
Controller

CHARGE
ENABLE

CSIP

CSIN

BOOT

UGATE

PHASE

LGATE

PGND

CSOP

CSON

CELLS

ICM

GND

DCIN

ACSET

VDDP

VDD

ACPRN

ICOMP

VCOMP

VADJ

EN

CHLIM

ACLIM

VREF

Q1

C8

0.1µF

ISL6251,

ISL6251A

FLOATING

4.2V/CELL

C7

D3

R7: 100Ω

CHARGE
ENABLE

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ISL6251, ISL6251A

DCIN

ACSET

VDDP

VDD

ACPRN

CHLIM

EN

ICM

ACLIM

VREF

ICOMP

VCOMP

VADJ

Q1

Q2

L
10µH

C1
10µF

C10
10µF

R1
40mΩ

AC ADAPTER

R2
20mΩ

R4

2.2Ω

R5
100k

R8

130k

1%

R9

10.2k,1%
C2

0.1µF

C4

0.1µF

C3
1µF

6.8nF

C9
1µF

C8

0.1µF

ISL6251

ISL6251A

C5

10nF

FLOATING

4.2V/CELL

R6

10k

C7
1µF

R10

4.7Ω

Battery

Pack

BAT+

SCL
SDL
TEMP
BAT-

SCL
SDL

A/D INPUT

GND

5.15A INPUT

CURRENT LIMIT

3 CELLS

HOST

R11, R12, R13
10k

D1

Optional

VDDP

D2

D3

R7: 100Ω

CSIP

CSIN

BOOT

UGATE

PHASE

LGATE

PGND

CSOP

CSON

CELLS

GND

C11
3300pF

D4

SYSTEM LOAD

DCIN

ACSET

VDDP

VDD

ACPRN

CHLIM

EN

ICM

ACLIM

VREF

ICOMP

VCOMP

VADJ

R3: 18Ω

C6

ISL6251,

ISL6251A

Battery

Pack

BAT+

SCL
SDL
TEMP
BAT-

Battery

Pack

BAT+

SCL
SDL
TEMP
BAT-

VCC

OUTPUT

D/A OUTPUT

A/D INPUT

DIGITAL

INPUT

AVDD/VREF

CSIP

CSIN

BOOT

UGATE

PHASE

LGATE

PGND

CSOP

CSON

CELLS

GND

FIGURE 13. ISL6251, ISL6251A T YPICAL APPLICATION CIRCUIT WITH MICRO-CONTROLLER

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ISL6251, ISL6251A

V 3.99V 175.0V

VADJCELL

+=

(EQ. 1)

V

VADJ

VREF

R

bot_VADJ

514k

||

R

top_VADJ 514k R

bot_VADJ

514k

||

+

||

-------------------------------------------------------------------------------------------------

×

=

(EQ. 2)

Theory of Operation

Introduction

The ISL6251, ISL6251A includes all of the functions necessary to
charge 2 to 4 cell Li-Ion and Li-polymer batteries. A high
efficiency synchronous buck converter is used to control the
charging voltage and charging current up to 10A. The ISL6251,
ISL6251A has input current limiting and analog inputs for setting
the charge current and charge voltage; CHLIM inputs are used to
control charge current and VADJ inputs are used to control
charge voltage.

The ISL6251, ISL6251A charges the battery with constant
charge current, set by the CHLIM input, until the battery voltage
rises up to a programmed charge voltage set by VADJ input; then
the charger begins to operate at a constant voltage charge
mode.

The EN input allows shutdown of the charger through a
command from a micro-controller. It also uses EN to safely
shutdown the charger when the battery is in extremely hot
conditions. The amount of adapter current is reported on the ICM
output. Figure 11 shows the IC functional block diagram.

The synchronous buck converter uses external N-channel
MOSFETs to convert the input voltage to the required charging
current and charging voltage. Figure 12 shows the ISL6251,
ISL6251A typical application circuit with charging current and
charging voltage fixed at specific values. The typical application
circuit shown in Figure 13 shows the ISL6251, ISL6251A typical
application circuit, which uses a micro-controller to adjust the
charging current set by CHLIM input. The voltage at CHLIM and
the value of R1 sets the charging current. The DC/DC converter
generates the control signals to drive two external N-channel
MOSFETs to regulate the voltage and current set by the ACLIM,
CHLIM, VADJ and CELLS inputs.

The ISL6251, ISL6251A features a voltage regulation loop
(VCOMP) and two current regulation loops (ICOMP). The VCOMP
voltage regulation loop monitors CSON to ensure that its voltage
never exceeds the voltage and regulates the battery charge
voltage set by VADJ. The ICOMP current regulation loops regulate
the battery charging current delivered to the battery to ensure
that it never exceeds the charging current limit set by CHLIM; and
the ICOMP current regulation loops also regulate the input
current drawn from the AC adapter to ensure that it never
exceeds the input current limit set by ACLIM, and to prevent a
system crash and AC adapter overload.

PWM Control

The ISL6251, ISL6251A employs a fixed frequency PWM current
mode control architecture with a feed forward function. The
feed-forward function maintains a constant modulator gain of 11
to achieve fast line regulation as the buck input voltage changes.
When the battery charge voltage approaches the input voltage,
the DC/DC converter operates in dropout mode, where there is a
timer to prevent the frequency from dropping into the audible
frequency range. It can achieve a duty cycle of up to 99.6%.

To prevent boosting of the system bus voltage, the battery
charger operates in standard-buck mode when CSOP-CSON
drops below 4.25mV. Once in standard-buck mode, hysteresis

does not allow synchronous operation of the DC/DC converter
until CSOP-CSON rises above 12.5mV.

An adaptive gate drive scheme is used to control the dead time
between two switches. The dead time control circuit monitors the
LGATE output and prevents the upper side MOSFET from turning
on until LGATE is fully off, preventing cross-conduction and
shoot-through. In order for the dead time circuit to work properly,
there must be a low resistance, low inductance path from the
LGATE driver to MOSFET gate, and from the source of MOSFET to
PGND. The external Schottky diode is between the VDDP pin and
BOOT pin to keep the bootstrap capacitor charged.

Setting the Battery Regulation Voltage

The ISL6251, ISL6251A uses a high-accuracy trimmed band-gap
voltage reference to regulate the battery charging voltage. The
VADJ input adjusts the charger output voltage, and the VADJ
control voltage can vary from 0 to VREF, providing a 10%
adjustment range (from 4.2V -5% to 4.2V +5%) on CSON
regulation voltage. An overall voltage accuracy of better than

0.5% is achieved.

The per-cell battery termination voltage is a function of the
battery chemistry. Consult the battery manufacturers to
determine this voltage.

• Float VADJ to set the battery voltage V
the cells,

• Connect VADJ to VREF to set 4.41V × number of cells,

• Connect VADJ to ground to set 3.99V × number of the cells.

So, the maximum battery voltage of 17.6V can be achieved. Note
that other battery charge voltages can be set by connecting a
resistor divider from VREF to ground. The resistor divider should
be sized to draw no more than 100µA from VREF; or connect a
low impedance voltage source like the D/A converter in the
micro-controller. The programmed battery voltage per cell can be
determined by Equation 1:

An external resistor divider from VREF sets the voltage at VADJ
according to Equation 2:

where R

bot_VADJ

and R

top_VADJ

are external resistors at VADJ. To
minimize accuracy loss due to interaction with VADJ’s internal
resistor divider, ensure the AC resistance looking back into the
external resistor divider is less than 25k.

Connect CELLS as shown in Table 1 to charge 2, 3 or 4 Li+ cells.
When charging other cell chemistries, use CELLS to select an
output voltage range for the charger. The internal error amplifier
gm1 maintains voltage regulation. The voltage error amplifier is
compensated at VCOMP. The component values shown in
Figure 12 provide suitable performance for most applications.
Individual compensation of the voltage regulation and current­regulation loops allows for optimal compensation.

= 4.2V × number of

CSON

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12

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March 13, 2014

ISL6251, ISL6251A

I

CHG

165mV

R

1

-------------------

V

CHLIM

3.3V

--------------------- -

=

(EQ. 3)

I

CHG

165mV

R

1

-------------------

V

CHLIM trickle,

3.3V

--------------------------------------- -

=

(EQ. 4)

⎟
⎠

⎞

⎜
⎝

⎛

+= 050.0V

VREF

05.0

R

1

I

ACLIM

2

INPUT

(EQ. 5)

V

ACLIM

VREF

R

bot_ACLIM

152k

||

R

top_ACLIM 152k R

bot_ACLIM

152k

||

+

||

------------------------------------------------------------------------------------------------------

×

=

(EQ. 6)

ACSET

9

8

rise,th

V1

R

R

V •

⎟

⎟
⎠

⎞

⎜

⎜
⎝

⎛

+=

(EQ. 7)

8hysACSET

9

8

fall,th

RIV1

R

R

V −•

⎟

⎟
⎠

⎞

⎜

⎜
⎝

⎛

+=

(EQ. 8)

TABLE 1. CELL NUMBER PROGRAMMING

CELLS CELL NUMBER

VDD 4

GND 3

Float 2

Setting the Battery Charge Current Limit

The CHLIM input sets the maximum charging current. The current
set by the current sense-resistor connects between CSOP and
CSON. The full-scale differential voltage between CSOP and
CSON is 165mV for CHLIM = 3.3V, so the maximum charging
current is 4.125A for a 40mΩ sensing resistor. Other battery
charge current-sense threshold values can be set by connecting a
resistor divider from VREF or 3.3V to ground, or by connecting a
low impedance voltage source like a D/A converter in the
micro-controller. Unlike VADJ and ACLIM, CHLIM does not have an
internal resistor divider network. The charge current limit
threshold is given by Equation 3:

To set the trickle charge current for the dumb charger, a resistor
in series with a switch Q3 (Figure 12) controlled by the micro­controller is connected from CHLIM pin to ground. The trickle
charge current is determined by Equation 4:

When the CHLIM voltage is below 88mV (typical), it will disable
the battery charger. When choosing the current sensing resistor,
note that the voltage drop across the sensing resistor causes
further power dissipation, reducing efficiency. However, adjusting
CHLIM voltage to reduce the voltage across the current sense
resistor R1 will degrade accuracy due to the smaller signal to the
input of the current sense amplifier. There is a trade-off between
accuracy and power dissipation. A low pass filter is
recommended to eliminate switching noise. Connect the resistor
to the CSOP pin instead of the CSON pin, as the CSOP pin has
lower bias current and less influence on current-sense accuracy
and voltage regulation accuracy.

Setting the Input Current Limit

The total input current from an AC adapter, or other DC source, is
a function of the system supply current and the battery-charging
current. The input current regulator limits the input current by
reducing the charging current, when the input current exceeds
the input current limit set point. System current normally
fluctuates as portions of the system are powered up or down.
Without input current regulation, the source must be able to
supply the maximum system current and the maximum charger
input current simultaneously. By using the input current limiter,
the current capability of the AC adapter can be lowered, reducing
system cost.

The ISL6251, ISL6251A limits the battery charge current when
the input current-limit threshold is exceeded, ensuring the
battery charger does not load down the AC adapter voltage. This
constant input current regulation allows the adapter to fully

power the system and prevent the AC adapter from overloading
and crashing the system bus.

An internal amplifier gm3 compares the voltage between CSIP
and CSIN to the input current limit threshold voltage set by
ACLIM. Connect ACLIM to REF, Float and GND for the full-scale
input current limit threshold voltage of 100mV, 75mV and 50mV,
respectively, or use a resistor divider from VREF to ground to set
the input current limit as shown in Equation 5:

An external resistor divider from VREF sets the voltage at ACLIM
according to Equation 6:

where R

bot_ACLIM

and R

top_ACLIM

are external resistors at
ACLIM. To minimize accuracy loss due to interaction with ACLIM’s
internal resistor divider, ensure the AC resistance looking back into
the external resistor divider is less than 25k.

When choosing the current sense resistor, note that the voltage
drop across this resistor causes further power dissipation,
reducing efficiency. The AC adapter current sense accuracy is
very important. Use a 1% tolerance current-sense resistor. The
highest accuracy of ±3% is achieved with 100mV current-sense
threshold voltage for ACLIM = VREF, but it has the highest power
dissipation. For example, it has 400mW power dissipation for
rated 4A AC adapter and 1W sensing resistor may have to be
used. ±4% and ±6% accuracy can be achieved with 75mV and
50mV current-sense threshold voltage for ACLIM = Floating and
ACLIM = GND, respectively.

A low pass filter is suggested to eliminate the switching noise.
Connect the resistor to CSIN pin instead of CSIP pin because
CSIN pin has lower bias current and less influence on the
current-sense accuracy.

AC Adapter Detection

Connect the AC adapter voltage through a resistor divider to
ACSET to detect when AC power is available, as shown in
Figure 12. ACPRN is an open-drain output and is high when
ACSET is less than V
V

th,fall

Where I

. V

and V

th,rise

is the ACSET input bias current hysteresis and V

hys

, and active low when ACSET is above

th,rise

are given by Equations 7 and 8:

th,fall

ACSET

= 1.24V (min), 1.26V (typ) and 1.28V (max). The hysteresis is
I

hysR8

, where I

= 2.2µA (min), 3.4µA (typ) and 4.4µA (max).

hys

Current Measurement

Use ICM to monitor the input current being sensed across CSIP
and CSIN. The output voltage range is 0 to 2.5V. The voltage of
ICM is proportional to the voltage drop across CSIP and CSIN, and

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ISL6251, ISL6251A

ICM 19.9 I

INPUTR2

••=

(EQ. 9)

sMAX,IN

BAT

L

BATMAX,IN

f V

V

I

VV

L

Δ

−

=

(EQ. 10)

MAXBAT,L

I30%I ⋅=

Δ

(EQ. 11)

LMAX,BATPeak

I

2

1

II

Δ

+=

(EQ. 12)

()

D1 D

f L 12

V

I

s

MAX,IN

RMS

−=

(EQ. 13)

is given by Equation 9:

where I

is the DC current drawn from the AC adapter. ICM

INPUT

has ±3% accuracy.

A low pass filter connected to ICM output is used to filter the
switching noise.

LDO Regulator

VDD provides a 5.075V supply voltage from the internal LDO
regulator from DCIN and can deliver up to 30mA of current. The
MOSFET drivers are powered by VDDP, which must be connected
to VDDP as shown in Figure 12. VDDP connects to VDD through
an external resistor. Bypass VDDP and VDD with a 1µF capacitor.

Shutdown

The ISL6251, ISL6251A features a low-power shutdown mode.
Driving EN low shuts down the charger. In shutdown, the DC/DC
converter is disabled, and VCOMP and ICOMP are pulled to
ground. The ICM, ACPRN outputs continue to function.

EN can be driven by a thermistor to allow automatic shutdown
when the battery pack is hot. Often a NTC thermistor is included
inside the battery pack to measure its temperature. When
connected to the charger, the thermistor forms a voltage divider
with a resistive pull-up to the VREF. The threshold voltage of EN is

1.06V with 60mV hysteresis. The thermistor can be selected to
have a resistance vs temperature characteristic that abruptly
decreases above a critical temperature. This arrangement
automatically shuts down the charger when the battery pack is
above a critical temperature.

Another method for inhibiting charging is to force CHLIM below
88mV (typ).

Short Circuit Protection and 0V Battery
Charging

Since the battery charger will regulate the charge current to the
limit set by CHLIM, it automatically has short circuit protection
and is able to provide the charge current to wake up an extremely
discharged battery.

Over-Temperature Protection

If the die temp exceeds +150°C, it stops charging. Once the die
temp drops below +125°C, charging will start up again.

Application Information

The following battery charger design refers to the typical
application circuit in Figure 12, where typical battery
configuration of 4S2P is used. This section describes how to
select the external components including the inductor, input and
output capacitors, switching MOSFETs, and current sensing
resistors.

Inductor Selection

The inductor selection has trade-offs between cost, size and
efficiency. For example, the lower the inductance, the smaller the
size, but ripple current is higher. This also results in higher AC

losses in the magnetic core and the windings, which decrease
the system efficiency. On the other hand, the higher inductance
results in lower ripple current and smaller output filter
capacitors, but it has higher DCR (DC resistance of the inductor)
loss, and has slower transient response. So, the practical
inductor design is based on the inductor ripple current being
±(15-20)% of the maximum operating DC current at maximum
input voltage. The required inductance can be calculated from
Equation 10:

, V

Where V

IN,MAX

, and fs are the maximum input voltage,

BAT

battery voltage and switching frequency, respectively. The
inductor ripple current

ΔI is found from Equation 11:

where the maximum peak-to-peak ripple current is 30% of the
maximum charge current is used.

For V

IN,MAX

= 19V, V

= 16.8V, I

BAT

BAT,MAX

= 2.6A, and
fs= 300kHz, the calculated inductance is 8.3µH. Choosing the
closest standard value gives L = 10µH. Ferrite cores are often the
best choice since they are optimized at 300kHz to 600kHz
operation with low core loss. The core must be large enough not
to saturate at the peak inductor current I

Peak

:

Output Capacitor Selection

The output capacitor in parallel with the battery is used to absorb
the high frequency switching ripple current and smooth the
output voltage. The RMS value of the output ripple current I
given by Equation 13:

where the duty cycle D is the ratio of the output voltage (battery
voltage) over the input voltage for continuous conduction mode
which is typical operation for the battery charger. During the
battery charge period, the output voltage varies from its initial
battery voltage to the rated battery voltage. So, the duty cycle
change can be in the range of between 0.53 and 0.88 for the
minimum battery voltage of 10V (2.5V/Cell) and the maximum
battery voltage of 16.8V.

For V

= 19V, VBAT = 16.8V, L = 10µH, and fs= 300kHz, the

IN,MAX

maximum RMS current is 0.19A. A typical 10F ceramic capacitor
is a good choice to absorb this current and also has very small
size. The tantalum capacitor has a known failure mechanism
when subjected to high surge current.

EMI considerations usually make it desirable to minimize ripple
current in the battery leads. Beads may be added in series with
the battery pack to increase the battery impedance at 300kHz
switching frequency. Switching ripple current splits between the
battery and the output capacitor depending on the ESR of the
output capacitor and battery impedance. If the ESR of the output

rms

is

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ISL6251, ISL6251A

DSON

2
BAT

IN

OUT

Conduction,1Q

RI

V

V

P =

(EQ. 14)

s INrr

ksin,g

gd

sLPIN

source,g

gd

sLVINSwitching,1Q

fVQ

I

Q

f IV

2

1

I

Q

f IV

2

1

P ++=

(EQ. 15)

DSON

2

BAT

IN

OUT

2Q

RI

V

V

1P

⎟

⎟
⎠

⎞

⎜

⎜
⎝

⎛

−=

(EQ. 16)

s

GATE

GATE

f

I

Q ≤

(EQ. 17)

()

IN

OUTINOUT

BATrms

V

VVV

II−=

(EQ. 18)

capacitor is 10mΩ and battery impedance is raised to 2Ω with a
bead, then only 0.5% of the ripple current will flow in the battery.

MOSFET Selection

The Notebook battery charger synchronous buck converter has
the input voltage from the AC adapter output. The maximum AC
adapter output voltage does not exceed 25V. Therefore, 30V logic
MOSFET should be used.

The high side MOSFET must be able to dissipate the conduction
losses plus the switching losses. For the battery charger
application, the input voltage of the synchronous buck converter
is equal to the AC adapter output voltage, which is relatively
constant. The maximum efficiency is achieved by selecting a
high side MOSFET that has the conduction losses equal to the
switching losses. Ensure that ISL6251, ISL6251A LGATE gate
driver can supply sufficient gate current to prevent it from
conduction, which is due to the injected current into the drain-to­source parasitic capacitor (Miller capacitor C
the voltage rising rate at phase node at the time instant of the
high-side MOSFET turning on; otherwise, cross-conduction
problems may occur. Reasonably slowing turn-on speed of the
high-side MOSFET by connecting a resistor between the BOOT pin
and gate drive supply source, and the high sink current capability
of the low-side MOSFET gate driver help reduce the possibility of
cross-conduction.

For the high-side MOSFET, the worst-case conduction losses
occur at the minimum input voltage:

), and caused by

gd

For the low-side MOSFET, the worst-case power dissipation
occurs at minimum battery voltage and maximum input voltage:

Choose a low-side MOSFET that has the lowest possible on­resistance with a moderate-sized package like the SO-8 and is
reasonably priced. The switching losses are not an issue for the
low side MOSFET because it operates at zero-voltage-switching.

Choose a Schottky diode in parallel with low-side MOSFET Q2
with a forward voltage drop low enough to prevent the low-side
MOSFET Q2 body-diode from turning on during the dead time.
This also reduces the power loss in the high-side MOSFET
associated with the reverse recovery of the low-side MOSFET Q2
body diode.

As a general rule, select a diode with DC current rating equal to
one-third of the load current. One option is to choose a combined
MOSFET with the Schottky diode in a single package. The
integrated packages may work better in practice because there
is less stray inductance due to a short connection. This Schottky
diode is optional and may be removed if efficiency loss can be
tolerated. In addition, ensure that the required total gate drive
current for the selected MOSFETs should be less than 24mA. So,
the total gate charge for the high-side and low-side MOSFETs is
limited by Equation 17:

The optimum efficiency occurs when the switching losses equal
the conduction losses. However, it is difficult to calculate the
switching losses in the high-side MOSFET since it must allow for
difficult-to-quantify factors that influence the turn-on and turn-off
times. These factors include the MOSFET internal gate
resistance, gate charge, threshold voltage, stray inductance, pull­up and pull-down resistance of the gate driver. The following
switching loss calculation provides a rough estimate.

Where Q

: drain-to-gate charge, Qrr: total reverse recovery

gd

charge of the body-diode in low side MOSFET, ILV: inductor valley
current, I

Inductor peak current, I

LP:

g,sink

and Ig,

source

are the

peak gate-drive source/sink current of Q1, respectively.

To achieve low switching losses, it requires low drain-to-gate
charge Q

. Generally, the lower the drain-to-gate charge, the

gd

higher the on-resistance. Therefore, there is a trade-off between
the on-resistance and drain-to-gate charge. Good MOSFET
selection is based on the Figure of Merit (FOM), which is a
product of the total gate charge and on-resistance. Usually, the
smaller the value of FOM, the higher the efficiency for the same
application.

Where I
than 24mA. Substituting I

is the total gate drive current and should be less

GATE

=24mA and fs= 300kHz into the

GATE

above equation yields that the total gate charge should be less
than 80nC. Therefore, the ISL6251, ISL6251A easily drives the
battery charge current up to 10A.

Input Capacitor Selection

The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by Equation 18:

This RMS ripple current must be smaller than the rated RMS
current in the capacitor datasheet. Non-tantalum chemistries
(ceramic, aluminum, or OSCON) are preferred due to their
resistance to power-up surge currents when the AC adapter is
plugged into the battery charger. For Notebook battery charger
applications, it is recommend that ceramic capacitors or polymer
capacitors from Sanyo be used due to their small size and
reasonable cost.

Table 2 shows the component lists for the typical application
circuit in Figure 12.

TABLE 2. COMPO NENT LIST

PARTS PART NUMBERS AND MANUFACTURER

C1, C10 10

C2, C4, C8 0.1

μF/25V ceramic capacitor, Taiyo Yuden

TMK325 MJ106MY X5R (3.2x2.5x1.9mm)

μF/50V ceramic capacitor

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15

FN9202.3

March 13, 2014

ISL6251, ISL6251A

M

11

V

IN

---------

.=

(EQ. 19)

()

1

Q

SS

S

1

V

d

ˆ

v

ˆ

SF

po

2

o

2

esr

in

o

1

++

+

==

ω

ω

ω

(EQ. 20)

,

CR

1

oc

esr

=

ω

L

C

RQ

o

op

≈

o

o

LC

1

=

ω

()

1

Q

SS

S

1

RR

V

d

ˆ

i

ˆ

SF

po

2
o

2

z

Lo

inL

2

++

+

+

==

ω

ω

ω

oo

z

CR

1

≈

ω

TiS() 0.25 RTF2S()M=

(EQ. 21)

TvS() KM F1S()AVS()=

(EQ. 22)

o

FB

V

V

K =

()

()

ST1

ST

)S(L

i

v

v

+

=

(EQ. 23)

LVS()

4

V

FB

V

O

-------------- -

ROR

L

+()

R

T

----------------------------- -

1

S

ω

esr

------------

+

1

S

ω

P

-------

+

------------------------

AVS()

ω

P

1

R

OCO

-----------------

≈

,=

(EQ. 24)

p

ω

TAB LE 2 . COMPONENT LIST (Continued)

PARTS PART NUMBERS AND MANUFACTURER

C3, C7, C9 1μF/10V ceramic capacitor, Taiyo Yuden

LMK212BJ105MG

C5 10nF ceramic capacitor

C6 6.8nF ceramic capacitor

C11 3300pF ceramic capacitor

D1 30V/3A Schottky diode, EC31QS03L (optional)

D2, D3 100mA/30V Schottky Diode, Central Semiconductor

D4 8A/30V Schottky rectifier, STPS8L30B (optional)

L10

Q1, Q2 30V/35m

Q3 Signal N-channel MOSFET, 2N7002

R1 40m

R2 20m

R3 18

R4 2.2

R5 100k

R6 10k,

R7 100

R8, R11 130k,

R9 10.2k

R10 4.7

R12 20k

R13 1.87k

μH/3.8A/26mΩ, Sumida, CDRH104R-100

Ω, FDS6912A, Fairchild.

Ω, ±1%, LRC-LR2512-01-R040-F, IRC

Ω, ±1%, LRC-LR2010-01-R020-F, IRC

Ω, ±5%, (0805)

Ω, ±5%, (0805)

Ω, ±5%, (0805)

±5%, (0805)

Ω, ±5%, (0805)

±1%, (0805)

Ω, ±1%, (0805)

Ω, ±5%, (0805)

Ω, ±1%, (0805)

Ω, ±1%, (0805)

Loop Compensation Design

ISL6251, ISL6251A uses constant frequency current mode
control architecture to achieve fast loop transient response. An
accurate current sensing resistor in series with the output
inductor is used to regulate the charge current, and the sensed
current signal is injected into the voltage loop to achieve current
mode control to simplify the loop compensation design. The
inductor is not considered as a state variable for current mode
control and the system becomes single order system. It is much
easier to design a compensator to stabilize the voltage loop than
voltage mode control. Figure 14 shows the small signal model of
the synchronous buck regulator.

Power Stage Transfer Functions

Transfer fun ction F1(S) from control to output voltage is:

Where ,

Transfer fun ction F2(S) from control to inductor current is:

, where .

Current loop gain Ti(S) is expressed as shown in Equation 21:

where R
equal to the product of the charging current sensing resistance
and the gain of the current sense amplifier, CA2. For ISL6251,
ISL6251A, R

The voltage gain with open current loop is:

Where , VFB is the feedback voltage of the voltage error

amplifier. The Voltage loop gain with current loop closed is given

by Equation 23:

If T

From the above equation, it is shown that the system is a single
order system, which has a single pole located at before the
half switching frequency. Therefore, simple type II compensator
can be easily used to stabilize the system.

is the trans-resistance in current loop. RT is usually

T

=20R1.

T

(S)>>1, then it can be simplified as follows:

i

PWM Comparator Gain Fm:

The PWM comparator gain Fm for peak current mode control is
given by Equation 19:

16

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FN9202.3

March 13, 2014

ISL6251, ISL6251A

FIGURE 14. SMALL SIGNAL MODEL OF SYNCHRONOUS BUCK

REGULATOR

dˆV

in

dˆI

L

in

v

ˆ

in

i

ˆ

L

+

1:D

+

L

i

ˆ

Co

Rc

Ro

-Av(S)

d

ˆ

comp

v

ˆ

R

T

11/Vin

+

T

i

(S)

K

o

v

ˆ

Tv(S)

dˆV

in

dˆI

L

in

v

ˆ

in

i

ˆ

L

+

1:D

+

L

i

ˆ

Rc

Ro

-Av(S)

d

ˆ

comp

v

ˆ

R

T

-

T

i

(S)

K

o

v

ˆ

Tv(S)

V

CA2

0.25V

CA2

dˆVindˆV

in

dˆILdˆI

L

in

v

ˆ

in

v

ˆ

in

i

ˆ

in

i

ˆ

L

+

1:D

+

L

i

ˆ

L

i

ˆ

Co

Rc

Ro

-Av(S)

dˆd

ˆ

comp

v

ˆ

comp

v

ˆ

R

T

11/Vin

+

T

i

(S)

K

o

v

ˆ

o

v

ˆ

Tv(S)

dˆVindˆV

in

dˆILdˆI

L

in

v

ˆ

in

v

ˆ

in

i

ˆ

in

i

ˆ

L

+

1:D

+

L

i

ˆ

L

i

ˆ

Rc

Ro

-Av(S)

dˆd

ˆ

comp

v

ˆ

comp

v

ˆ

R

T

-

T

i

(S)

K

o

v

ˆ

o

v

ˆ

Tv(S)

V

CA2

0.25V

CA2

()

SC

S

1

g

v

ˆ

v

ˆ

SA

1

cz

m

FB

comp

v

ω

+

==

(EQ. 25)

CR

1

11

cz

=

ω

-
+

R1

C1

V

REF

V

FB

Vo

g

m

V

COMP

-
+

Vo

­+

R1

C1

V

REF

V

FB

Vo

g

m

V

COMP

­+

Vo

FIGURE 15. VOLTAGE LOOP COMPENSATOR

s

f

20

1

5

1

⎟
⎠

⎞

⎜
⎝

⎛

−

()

oo

cz

CR

1

31 −=

ω

(EQ. 26)

R

1

8π fCVOCOR

T

gmV

FB

---------------------------------------

=

(EQ. 27)

cz 1

1

R

1

C

ω

=

(EQ. 28)

Figure 15 shows the voltage loop compensator, and its transfer
function is expressed as follows:

where

The loop gain T

(S) at cross over frequency of fc has unity gain.

v

Therefore, the compensator resistance R1 is determined by
Equation 27:

where g

is the trans-conductance of the voltage loop error

m

amplifier. Compensator capacitor C1 is then given by
Equation 28:

Example: V

= 19V, Vo= 16.8V, Io=2.6A, fs= 300kHz,

in

Co=10μF/10mΩ, L = 10μH, gm= 250μs, RT=0.8Ω,
VFB=2.1V, fc= 20kHz, then compensator resistance R1= 10kΩ.
Choose R
compensator capacitor is C
loop compensator: R

= 10kΩ. Put the compensator zero at 1.5kHz. The

1

1

= 6.5nF. Therefore, choose voltage

1

= 10k, C1=6.5nF.

PCB Layout Considerations

Power and Signal Layers Placement on the
PCB

As a general rule, power layers should be close together, either
on the top or bottom of the board, with signal layers on the
opposite side of the board. As an example, layer arrangement on
a 4-layer board is shown below:

Compensator design goal:

•High DC gain

• Loop bandwidth f

:

c

• Gain margin: >10dB

•Phase margin: 40°

The compensator design procedure is as follows:

1. Put compensator zero at:

2. Put one compensator pole at zero frequency to achieve high
DC gain, and put another compensator pole at either ESR zero
frequency or half switching frequency, whichever is lower.

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17

1. Top Layer: signal lines, or half board for signal lines and the
other half board for power lines

2. Signal Ground

3. Power Layers: Power Ground

4. Bottom Layer: Power MOSFET, Inductors and other Power
traces

Separate the power voltage and current flowing path from the
control and logic level signal path. The controller IC will stay on
the signal layer, which is isolated by the signal ground to the
power signal traces.

Component Placement

The power MOSFET should be close to the IC so that the gate
drive signal, the LGATE, UGATE, PHASE, and BOOT, traces can be
short.

Place the components in such a way that the area under the IC
has less noise traces with high dv/dt and di/dt, such as gate
signals and phase node signals.

Signal Ground and Power Ground
Connection.

At minimum, a reasonably large area of copper, which will shield
other noise couplings through the IC, should be used as signal
ground beneath the IC. The best tie-point between the signal
ground and the power ground is at the negative side of the output
capacitor on each side, where there is little noise; a noisy trace
beneath the IC is not recommended.

FN9202.3

March 13, 2014

ISL6251, ISL6251A

GND and VDD Pin

At least one high quality ceramic decoupling cap should be used
to cross these two pins. The decoupling cap can be put close to
the IC.

LGATE Pin

This is the gate drive signal for the bottom MOSFET of the buck
converter. The signal going through this trace has both high dv/dt
and high di/dt, and the peak charging and discharging current is
very high. These two traces should be short, wide, and away from
other traces. There should be no other traces in parallel with
these traces on any layer.

PGND Pin

PGND pin should be laid out to the negative side of the relevant
output cap with separate traces. The negative side of the output
capacitor must be close to the source node of the bottom
MOSFET. This trace is the return path of LGATE.

PHASE Pin

This trace should be short, and positioned away from other weak
signal traces. This node has a very high dv/dt with a voltage
swing from the input voltage to ground. No trace should be in
parallel with it. This trace is also the return path for UGATE.
Connect this pin to the high-side MOSFET source.

UGATE Pin

This pin has a square shape waveform with high dv/dt. It
provides the gate drive current to charge and discharge the top
MOSFET with high di/dt. This trace should be wide, short, and
away from other traces similar to the LGATE.

BOOT Pin

This pin’s di/dt is as high as the UGATE; therefore, this trace
should be as short as possible.

CSOP, CSON Pins

The current sense resistor connects to the CSON and the CSOP
pins through a low pass filter. The CSON pin is also used as the
battery voltage feedback. The traces should be away from the
high dv/dt and di/di pins like PHASE, BOOT pins. In general, the
current sense resistor should be close to the IC. Other layout
arrangements should be adjusted accordingly.

EN Pin

This pin stays high at enable mode and low at idle mode and is
relatively robust. Enable signals should refer to the signal ground.

DCIN Pin

This pin connects to AC adapter output voltage, and should be
less noise sensitive.

Copper Size for the Phase Node

The capacitance of PHASE should be kept very low to minimize
ringing. It would be best to limit the size of the PHASE node
copper in strict accordance with the current and thermal
management of the application.

Identify the Power and Signal Ground

The input and output capacitors of the converters, the source
terminal of the bottom switching MOSFET PGND should connect
to the power ground. The other components should connect to
signal ground. Signal and power ground are tied together at one
point.

Clamping Capacitor for Switching MOSFET

It is recommended that ceramic caps be used closely connected
to the drain of the high-side MOSFET, and the source of the low­side MOSFET. This capacitor reduces the noise and the power
loss of the MOSFET.

For additional products, see www.intersil.com/en/products.html

Intersil products are manufactured, assembled and tested utilizing ISO9001 quality systems as noted

in the quality certifications found at www.intersil.com/en/support/qualandreliability.html

Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time
without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be
accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.

For information regarding Intersil Corporation and its products, see www.intersil.com

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18

FN9202.3

March 13, 2014

Package Outline Drawing

located within the zone indicated. The pin #1 identifier may be

Unless otherwise specified, tolerance : Decimal ± 0.05

Tiebar shown (if present) is a non-functional feature.

The configuration of the pin #1 identifier is optional, but must be

between 0.15mm and 0.30mm from the terminal tip.

Dimension b applies to the metallized terminal and is measured

Dimensions in ( ) for Reference Only.

Dimensioning and tolerancing conform to AMSE Y14.5m-1994.

6.

either a mold or mark feature.

3.

5.

4.

2.

Dimensions are in millimeters.1.

NOTES:

BOTTOM VIEW

DETAIL "X"

TYPICAL RECOMMENDED LAND PATTERN

TOP VIEW

BOTTOM VIEW

SIDE VIEW

5.00

A

5.00

B

INDEX AREA

PIN 1

6

(4X) 0.15

28X 0.55 ± 0.10 4

A

28X 0.25

M0.10 C B

14

8

4X

0.50

24X

3.0

6

PIN #1 INDEX AREA

3 .10 ± 0 . 15

0 . 90 ± 0.1

BASE PLANE

SEE DETAIL "X"

SEATING PLANE

0.10

C

C

0.08 C

0 . 2 REF

C

0 . 05 MAX.

0 . 00 MIN.

5

( 3. 10)

( 4. 65 TYP )

( 24X 0 . 50)

(28X 0 . 25 )

( 28X 0 . 75)

15

22

21

7

1

28

+ 0.05

- 0.07

L28.5x5

28 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE
Rev 2, 10/07

ISL6251, ISL6251A

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19

FN9202.3

March 13, 2014

ISL6251, ISL6251A

NOTES:

1. Symbols are defined in the “MO Series Symbol List” in Section 2.2 of Publication
Number 95.

2. Dimensioning and tolerancing per ANSI Y14.5M-1982.

3. Package length does not include mold flash, protrusions or gate burrs. Mold
flash,
protrusion and gate burrs shall not exceed 0.15mm (0.006 inch) per side.

4. Package width does not include interlead flash or protrusions. Interlead flash and
protrusions shall not exceed 0.25mm (0.010 inch) per side.

5. The chamfer on the body is optional. If it is not present, a visual index feature
must be located within the crosshatched area.

6. Terminal numbers are shown for reference only.

7. Lead width does not include dambar protrusion. Allowable dambar protrusion
shall be 0.10mm (0.004 inch) total in excess of “B” dimension at maximum mate­rial condition.

8. Controlling dimension: MILLIMETER.

INDEX

AREA

24

1

-B-

0.17(0.007)

CAM

BS

-A-

M

-C-

SEATING PLANE

0.10(0.004)
x 45°

0.25

0.010

GAUGE

PLANE

3.98

3.81

6.19

5.80

4

0.25(0.010) B

M

M

1.27

0.41

0.49

0.26

5

8°
0°

1.54

0.25

0.18

8.74

8.55

3

1.75

1.35

0.25

0.10

0.30

0.20

7

0.635 BSC

5.59

4.06

7.11

0.635

DETAIL “X”

SIDE VIEW 1

TYPICAL RECOMMENDED LAND PATTERN

TOP VIEW

SIDE VIEW 2

5

0.38

Package Outline Drawing

M24.15

24 LEAD SHRINK SMALL OUTLINE PLASTIC PACKAGE (QSOP/SSOP)

0.150” WIDE BODY

Rev 3, 2/13

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20

FN9202.3

March 13, 2014