LINEAR TECHNOLOGY LTC1876 Technical data

FEATURES

LTC1876

High Efficiency, 2-Phase,

Dual Synchronous Step-Down Switching

Controller and Step-Up Regulator

U

DESCRIPTIO

Step-Down Controller

■

Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise

■

Power Good Output Voltage Indicator

■

OPTI-LOOPTM Compensation Minimizes C

■

DC Programmed Fixed Frequency 150kHz to 300kHz

■

Wide VIN Range: 3.5V to 36V Operation

■

Very Low Dropout Operation: 99% Duty Cycle

■

Adjustable Soft-Start Current Ramping

■

Latched Short-Circuit Shutdown with Defeat Option

■

Remote Output Voltage Sense and OV Protection

■

5V and 3.3V Standby Regulators

■

Selectable Const. Freq. or Burst Mode

OUT

TM

Operation

Step-Up Regulator

■

High Operating Switching Frequency of 1.2MHz

■

Low Internal V

■

Wide VIN Range: 2.6V to 16V Operation

■

High Output Voltage: Up to 34V

Switch: 400mV @ 1A, V

CESAT

IN

= 3V

U

APPLICATIO S

■

3.3V Input Step-Down Converter

■

Notebook and Palmtop Computers, PDAs

■

Battery-Operated Digital Devices

The LTC®1876 is a high performance triple output switching
regulator. It incorporates a dual step-down switching con­troller that drives all N-channel synchronous power MOSFET
stages. A step-up regulator with an internal 1A, 36V switch
provides the third output.

The step-down controllers minimize power loss and noise
by operating the output stage of each controller out of
phase. OPTI-LOOP compensation allows the transient
response to be optimized over a wide range of output
capacitance and ESR values. A RUN/SS pin for each
controller provides both soft-start and an optional timed,
short-circuit shutdown that can be configured to latch off
one or both controllers. Current foldback provides
additional short-circuit protection. In an overvoltage
condition, the bottom MOSFET is latched on until V

OUT

returns to normal. The FCB pin can be used to inhibit Burst
Mode operation or to enable regulation of a secondary
output voltage.

The step-up regulator operates at 1.2MHz, allowing the
use of tiny low cost capacitors and inductors. In addition,
its internal 1A switch allows high current outputs to be
generated. Its current mode control scheme provides
excellent line and load regulation.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.

TYPICAL APPLICATIO

V

IN

5.2V

TO 28V

V

OUT2

3.3V

0.01Ω

5A

33µF

35V

ALUM

+

10µF

35V
CER

6.3µH

63.4k

56µF
4V
SP

1%

+

Figure 1. High Efficiency Triple 5V/3.3V/12V Power Supply

U

10µH

220pF

15k

0.1µF

+

4.7µF
10V

1000pF

M1

M2

86.6k, 1%

105k

1%

10.2k

20k

1%

1%

M1, M2, M3, M4: FDS6680A

6.8µH

10µF

20V

1µF
CER

M3

M4

20k
1%

1000pF

0.1µF

15k

220pF

0.1µF

AUXVINV

INTV

CC

TG2 TG1

BOOST2 BOOST1
SW2 SW1

LTC1876

BG2 BG1

PGND

PGOOD

+

SENSE2

–

SENSE2
V

OSENSE2VOSENSE1

I

TH2

RUN/SS2 RUN/SS1

IN

AUXSW

AUXV

AUXSD

SENSE1
SENSE1

I

SGND

FB

+

–

TH1

0.1µF

+

0.01Ω

+

47µF

6.3V
SP

1876 TA01

V

OUT3

12V
200mA

V
5V
4A

OUT1

1876fa

1

LTC1876

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

Input Supply Voltage (VIN)......................... 36V to –0.3V

Topside Driver Voltages

(BOOST1, BOOST2) ...................................42V to –0.3V

Switch Voltage (SW1, SW2) .........................36V to –5V

INTV

(BOOST1-SW1), (BOOST2-SW2), ...............7V to –0.3V

SENSE1+, SENSE2+, SENSE1–, SENSE2

FREQSET, STBYMD, FCB, PGOOD

I

TH1, ITH2

Peak Output Current <10µs (TG1, TG2, BG1, BG2) ... 3A

INTVCC Peak Output Current ................................ 50mA

AUXV

AUXSD..................................................................... 10V

AUXSW..................................................... 36V to –0.3V

AUXVFB Voltage ....................................... 2.5V to –0.3V

Current into AUXV
Operating Temperature Range (Note 2) ...–40°C to 85°C

Junction Temperature (Note 3)............................. 125°C

Storage Temperature Range ..................–65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

EXTVCC, RUN/SS1, RUN/SS2, PGOOD,

CC,

–

Voltages ...................................(1.1)INTVCC to –0.3V

Voltages ..................................................7V to –0.3V

, V

OSENSE1

..................................................................

IN

, V

OSENSE2

Voltages ... 2.7V to –0.3V

16V to –0.3V

.......................................................

FB

±1mA

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

RUN/SS1
SENSE1
SENSE1

V

OSENSE1

FREQSET
STBYMD

FCB

I

TH1

SGND

3.3V

OUT

I

ITH2

V

OSENSE2

SENSE2
SENSE2

AUXSGND

AUXV
AUXSW
AUXSW

1

+

2

–

3
4
5
6
7
8

9
10
11
12

–

13

+

14
15
16

FB

17
18

G PACKAGE

36-LEAD PLASTIC SSOP

T

= 125°C, θJA = 95°C/W

JMAX

36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19

PGOOD
TG1
SW1
BOOST1
V

IN

BG1
EXTV

CC

INTV

CC

PGND
BG2
BOOST2
SW2
TG2
RUN/SS2
AUXSD
AUXV

IN

AUXPGND
AUXPGND

Consult factory for parts specified with wider operating temperature ranges.

ORDER PART

NUMBER

LTC1876EG

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, V

The ● denotes the specifications which apply over the full operating

RUN/SS1, 2

= 5V, AUXVIN = 3V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Main Control Loops

V

OSENSE1, 2

I

VOSENSE1, 2

V

REFLNREG

V

LOADREG

g

m1, 2

g

mOL1, 2

I

Q

V

FCB

I

FCB

V

BINHIBIT

Regulated Feedback Voltage I

Voltage = 1.2V (Note 4) ● 0.792 0.800 0.808 V

TH1, 2

Feedback Current (Note 4) –5 –50 nA
Reference Voltage Line Regulation VIN = 3.6V to 30V (Note 4) 0.002 0.02 %/V
Output Voltage Load Regulation (Note 4)

Voltage = 1.2V to 0.7V ● 0.1 0.5 %

TH

Voltage = 1.2V to 2V ● –0.1 –0.5 %

TH

Transconductance Amplifier g

m

Transconductance Amplifier GBW I

Measured in Servo Loop; ∆I
Measured in Servo Loop; ∆I

I

= 1.2V; Sink/Source 5µA; (Note 4) 1.3 mmho

TH1, 2

= 1.2V; (Note 4) 3 MHz

TH1, 2

Input DC Supply Current (Note 5)

Normal Mode V
Standby V
Shutdown V

= 15V; EXTVCC Tied to V

IN
RUN/SS1, 2
RUN/SS1, 2

= 0V, V
= 0V, V

STBYMD
STBYMD

; V

OUT1

= 5V 350 µA

OUT1

> 2V 125 µA
= Open 20 35 µA

Forced Continuous Threshold ● 0.76 0.800 0.84 V
Forced Continuous Current V

= 0.85V –0.3 –0.18 –0.1 µA

FCB

Burst Inhibit (Constant Frequency) Measured at FCB pin 4.3 4.8 V
Threshold

1876fa

2

LTC1876

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, V

The ● denotes the specifications which apply over the full operating

RUN/SS1, 2

= 5V, AUXVIN = 3V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

UVLO Undervoltage Lockout VIN Ramping Down ● 3.5 4 V
V

OVL

I

SENSE

V

STBYMD

V

STBYMD

DF

MAX

I

RUN/SS1, 2

V

RUN/SS1, 2

V

RUN/SS1, 2

I

SCL1, 2

I

SDLHO

V

SENSE(MAX)

Overvoltage Feedback Threshold Measured at V

Sense Pins Total Source Current (Each Channel); V
MS Master Shutdown Threshold V
KA Keep-Alive Power On-Threshold V

OSENSE1, 2

SENSE1–, 2–

Ramping Down 0.4 0.6 V

STBYMD

Ramping Up, RUN

STBYMD

= V

SENSE1+, 2+

= 0V 1.5 2 V

SS1, 2

● 0.84 0.86 0.88 V

= 0V –85 –60 µA

Maximum Duty Factor In Dropout 98 99.4 %

Soft-Start Charge Current V

ON RUN/SS Pin ON Threshold V
LT RUN/SS Pin Latchoff Arming Threshold V

RUN/SS1, 2

RUN/SS1, VRUN/SS2

RUN/SS1, VRUN/SS2

RUN/SS Discharge Current Soft Short Condition V

V

RUN/SS1, 2

Shutdown Latch Disable Current V

Maximum Current Sense Threshold V

OSENSE1, 2

OSENSE1, 2

= 1.9V 0.5 1.2 µA

Rising 1.0 1.5 1.9 V
Rising from 3V 4.1 4.5 V

OSENSE1, 2

= 0.5V; 0.5 2 4 µA

= 4.5V

=0.5V 1.6 5 µA
= 0.7V, V

SENSE1–, 2–

= 5V ● 62 75 88 mV

TG Transition Time:

TG1, 2 t
TG1, 2 t

r
f

Rise Time C
Fall Time C

= 3300pF 50 90 ns

LOAD

= 3300pF 50 90 ns

LOAD

BG Transition Time:

BG1, 2 t
BG1, 2 t

TG/BG t

BG/TG t

t

ON(MIN)

r
f

1D

Rise Time C
Fall Time C

Top Gate Off to Bottom Gate On Delay

Synchronous Switch-On Delay Time C

2D

Bottom Gate Off to Top Gate On Delay

Top Switch-On Delay Time C

Minimum ON-Time Tested with a Square Wave (Note 7) 180 ns

= 3300pF 40 90 ns

LOAD

= 3300pF 40 80 ns

LOAD

= 3300pF Each Driver 90 ns

LOAD

= 3300pF Each Driver 90 ns

LOAD

INTVCC Linear Regulator

V

INTVCC

V

INT INTVCC Load Regulation ICC = 0 to 20mA, V

LDO

V

EXT EXTVCC Voltage Drop ICC = 20mA, V

LDO

V

EXTVCC

V

LDOHYS

Internal VCC Voltage 6V < VIN < 30V, V

EXTVCC

EXTVCC Switchover Voltage ICC = 20mA, EXTV

= 4V 4.8 5.0 5.2 V

EXTVCC

= 4V 0.2 1.0 %

EXTVCC

= 5V 80 160 mV

Ramping Positive ● 4.5 4.7 V

CC

EXTVCC Hysteresis 0.2 V

Oscillator

f

OSC

f

LOW

f

HIGH

I

FREQSET

Oscillator frequency V

Lowest Frequency V

Highest Frequency V

FREQSET Input Current V

= Open (Note 8) 190 220 250 kHz

FREQSET

= 0V 120 140 160 kHz

FREQSET

= 2.4V 280 310 360 kHz

FREQSET

= 2.4V –2 –1 µA

FREQSET

1876fa

3

LTC1876

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 15V, V

The ● denotes the specifications which apply over the full operating

RUN/SS1, 2

= 5V, AUXVIN = 3V unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

3.3V Linear Regulator

V

3.3OUT

V

3.3IL

V

3.3VL

PGOOD Output

V

PGL

I

PGOOD

V

PG

Aux Output

AUXV

INMIN

AUXV

FB

AUXI

FB

AUXI

Q

AUXV

LINEREG

AUXf

OSC

AUXDC

MAX

AUXI

LIMIT

AUXV

CESAT

AUXI

LEAKAGE

AUXV

AUXSD

I

AUXSD

3.3V Regulator Output Voltage No Load ● 3.25 3.35 3.45 V

3.3V Regulator Load Regulation I

3.3V Regulator Line Regulation 6V < V

PGOOD Voltage Low I
PGOOD Leakage Current V
PGOOD Trip Level, Either Controller V

= 0mA to 10mA 0.5 2 %

3.3

< 30V 0.05 0.2 %

IN

= 2mA 0.1 0.3 V

PGOOD

= 5V ±1 µA

PGOOD

with Respect to Set Output Voltage

OSENSE

Ramping Negative –6 –7.5 –9.5 %

V

OSENSE

Ramping Positive 6 7.5 9.5 %

V

OSENSE

AUX Minimum Operating Voltage ● 2.4 2.6 V
AUX Regulated Feedback Voltage ● 1.23 1.26 1.28 V
AUX Feedback Pin Bias Current ● 120 360 nA
AUX Input DC Supply Current

Normal Mode V
Shutdown V

= 2.4V, Not Switching 4 mA

AUXSD

= 0V 0.01 1 µA

AUXSD

AUX Line Regulation 2.6V ≤ AUXVIN ≤ 16V 0.01 0.05 %/V
AUX Oscillator Frequency ● 0.8 1.2 1.6 MHz
AUX Oscillator Maximum Duty Cycle ● 84 86 %
AUX Switch Current Limit (Note 9) 1 1.4 2 A
AUX Switch Saturation Voltage ISW = 900mA (Note 10) 330 550 mV
AUX Switch Leakage Current VSW = 5V 0.01 1 µA
AUX Shutdown Input Voltage

AUX Shutdown Upper Trip Point 2.4 V
AUX Shutdown Lower Trip Point 0.5 V

AUXSD Pin Bias Current
V
V

= 3V 16 32 µA

AUXSD

= 0V 0.01 0.1 µA

AUXSD

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LTC1876E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 3: T
dissipation P

is calculated from the ambient temperature TA and power

J

according to the following formulas:

D

LTC1876EG: TJ = TA + (PD • 95°C/W)
Note 4: The LTC1876 is tested in a feedback loop that servos V

specified voltage and measures the resultant V

OSENSE1, 2.

ITH1, 2

to a

4

Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.

Note 6: Rise and fall times are measured using 10% and 90% levels.
Delay times are measured using 50% levels.

Note 7: The minimum on-time condition is specified for an inductor peak­to-peak ripple current ≥40% of I

(see Minimum On-Time

MAX

Considerations in the Applications Information section).
Note 8: V

pin internally tied to 1.19V reference through a large

FREQSET

resistance.

Note 9: Current limit guaranteed by design and/or correlation to static test.
Note 10: 100% tested at wafer level.

1876fa

UW

TEMPERATURE (°C)

–50

INTV

CC

AND EXTV

CC

SWITCH VOLTAGE (V)

4.95

5.00

5.05

25 75

1876 G06

4.90

4.85

–25 0

50 100 125

4.80

4.70

4.75

INTVCC VOLTAGE

EXTVCC SWITCHOVER THRESHOLD

TYPICAL PERFOR A CE CHARACTERISTICS

LTC1876

Efficiency vs Output Current and
Mode (Figure 1)

100

Burst Mode

90

OPERATION

80
70
60
50
40

EFFICIENCY (%)

30
20
10

0

0.001

V

Supply Current vs Input

IN

0.01
OUTPUT CURRENT (A)

FORCED
CONTINUOUS
MODE

CONSTANT
FREQUENCY
(BURST DISABLE)

0.1

Voltage and Mode (Figure 1)

1000

800

600

400

SUPPLY CURRENT (µA)

200

0

05

BOTH
CONTROLLERS ON

STANDBY

SHUTDOWN

10

INPUT VOLTAGE (V)

20

15

1

25

VIN = 15V

= 5V

V

OUT

1876 G01

30

1876 G04

Efficiency vs Output Current
(Figure 1)

100

EFFICIENCY (%)

10

90

80

70

60

50

0.001

= 5V

V

OUT

VIN = 7V

VIN = 10V

V

= 15V

IN

V

= 20V

IN

0.01

0.1

OUTPUT CURRENT (A)

1

10

1876 G02

EXTVCC Voltage Drop

250

200

150

100

VOLTAGE DROP (mV)

CC

EXTV

50

35

0

10

0

CURRENT (mA)

30

40

20

50

1876 G05

Efficiency vs Input Voltage
(Figure 1)

100

90

80

70

EFFICIENCY (%)

60

50

5

INTV

15

INPUT VOLTAGE (V)

and EXTVCC Switch

CC

Voltage vs Temperature

V

= 5V

OUT

= 3A

I

OUT

25

35

1876 G03

5.1

5.0

4.9

4.8

VOLTAGE (V)

4.7

CC

INTV

4.6

4.5

4.4
0

Internal 5V LDO Line Regulation

I

= 1mA

LOAD

20 30 35

510

15 25

INPUT VOLTAGE (V)

1876 G07

Maximum Current Sense
Threshold vs Duty Factor

75

50

(mV)

SENSE

V

25

0

0

20 40 60 80

DUTY FACTOR (%)

1876 G08

100

Maximum Current Sense
Threshold vs Percent of Nominal
Output Voltage (Foldback)

80

70

60

50

(mV)

40

SENSE

V

30

20

10

0

0

PERCENT ON NOMINAL OUTPUT VOLTAGE (%)

25

50

75

100

1876 G09

1876fa

5

LTC1876

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Maximum Current Sense
Maximum Current Sense
Threshold vs V

80

V

SENSE(CM)

60

(mV)

40

SENSE

V

20

0

0

1234

= 1.6V

RUN/SS

V

RUN/SS

(V)

Load Regulation (Controller)

0.0

–0.1

(%)

OUT

–0.2

NORMALIZED V

–0.3

(Soft-Start)

56

1876 G10

FCB = 0V

= 15V

V

IN

FIGURE 1

Threshold vs Sense Common

Mode Voltage

80

76

72

(mV)

SENSE

68

V

64

(V)

ITH

V

60

2.5

2.0

1.5

1.0

0.5

1

0

COMMON MODE VOLTAGE (V)

V

vs V

ITH

RUN/SS

V

= 0.7V

OSENSE

2

3

Current Sense Threshold
vs I

Voltage

TH

90
80
70
60
50
40

(mV)

30
20

SENSE

V

10

0
–10
–20

4

5

1876 G11

–30

0.5

0

1.5

2

1

V

(V)

ITH

2.5

1876 G12

SENSE Pins Total Source Current

100

50

(µA)

0

SENSE

I

–50

–0.4

1

0

LOAD CURRENT (A)

Maximum Current Sense
Threshold vs Temperature

80

78

76

(mV)

SENSE

74

V

72

70

–50 –25

0

TEMPERATURE (°C)

6

3

4

2

5

1876 G13

0

0

234

1

V

RUN/SS

(V)

56

1876 G14

–100

0

24

V

COMMON MODE VOLTAGE (V)

SENSE

6

1876 G15

Current Sense Pin Input Current
vs Temperature

35

V

= 5V

OUT

33

31

29

27

CURRENT SENSE INPUT CURRENT (µA)

50

25

75

100

125

1876 G16

25

–50 –25

50

25

0

TEMPERATURE (°C)

100

125

1876 G17

75

RUN/SS Current vs Temperature

1.8

1.6

1.4

1.2

1.0

0.8

0.6

RUN/SS CURRENT (µA)

0.4

0.2

0

–50 –25

0 25 125
TEMPERATURE (°C)

75 10050

1876 G18

1876fa

TEMPERATURE (°C)

–50

UNDERVOLTAGE LOCKOUT (V)

3.40

3.45

3.50

25 75

1876 G21

3.35

3.30

–25 0

50 100 125

3.25

3.20

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LTC1876

EXTV

and Switch Resistance vs

CC

Temperature

10

8

6

4

SWITCH RESISTANCE (Ω)

CC

2

EXTV

0

–50 –25

0

TEMPERATURE (°C)

50

25

Shutdown Latch Thresholds vs
Temperature

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

SHUTDOWN LATCH THRESHOLDS (V)

0

–50 –25

LATCH ARMING

LATCHOFF

THRESHOLD

0 25 125
TEMPERATURE (°C)

Oscillator Frequency vs
Temperature (Controller)

350

300

250

200

150

FREQUENCY (kHz)

100

50

100

125

1876 G19

75

0

–50

–25 0

V

FREQSET

V

= OPEN

FREQSET

V

FREQSET

50 100 125

25 75

TEMPERATURE (

= 5V

= 0V

°C)

1876 G20

Undervoltage Lockout vs
Temperature (Controller)

Quiescent Current for Auxillary

Shutdown Pin Current (I

40

35

30

25

20

15

10

SHUTDOWN PIN CURRENT (µA)

5

0

12 4

75 10050

1876 G22

0

SHUTDOWN PIN VOLTAGE (V)

TA = 25°C

3

AUXVFB

T

= 100°C

A

5

)

1876 G23

4.6

4.5

4.4

4.3

4.2

4.1

4.0

3.9

QUIESCENT CURRENT (mA)

3.8

3.7

6

3.6

Regulator

= 1.3V

V

FB

NOT SWITCHING

–50

V

= 3.3V

IN

0

TEMPERATURE (°C)

50

VIN = 5V

100

1876 G24

Feedback Pin Voltage (AUXVFB)

1.28

1.27

1.26

1.25

1.24

FEEDBACK VOLTAGE (V)

1.23

1.22
–50

–25

02550

TEMPERATURE (°C)

75 100

1876 G25

Current Limit for Auxillary
Regulator

1.6

1.4

1.2

1.0

0.8

0.6

CURRENT LI MIT (A)

0.4

0.2

0

20

10

30

40

DUTY CYCLE (%)

Auxillary Regulator Switch
Oscillator Frequency

1.35

1.30

1.25

1.20

1.15

FREQUENCY (MHz)

1.10

1.05

50

60

70

80

1876 G26

90

–50 –30 –10

10 30 50 70 90 110

TEMPERATURE (°C)

1876 G28

1876fa

7

LTC1876

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Input Source/Capacitor

2A/DIV

V

200mV/DIV

V

SW1

10V/DIV

V

SW2

10V/DIV

Instantaneous Current (Figure 1)

I

IN

IN

200mV/DIV

Load Step (Figure 1)

V

OUT

I

OUT

2A/DIV

V

OUT

200mV/DIV

I

OUT

2A/DIV

Load Step (Figure 1)

V
V
I

OUT5

= 15V

IN
OUT

= 5V
= I

OUT3.3

1µs/DIV

= 2A

1876 G31

VIN = 15V

= 5V

V

OUT

LOAD STEP = 0A TO 3A
CONTINUOUS MODE

Burst Mode Operation (Figure 1)

V

OUT

20mV/DIV

I

OUT

0.5A/DIV

VIN = 15V

= 5V

V

OUT

= OPEN

V

FCB

I

= 20mA

OUT

10µs/DIV

1876 G32

UUU

PIN FUNCTIONS

RUN/SS1, RUN/SS2 (Pins 1, 23): Combination of Soft-Start,
Run Control Inputs and Short-Circuit Detection Timers. A
capacitor to ground at each of these pins sets the ramp time
to full output current. Forcing either of these pins back below
1V causes the IC to shut down the circuitry required for that
particular controller. Latchoff overcurrent protection is also
invoked via this pin as described in the Applications Informa­tion section.

SENSE1+, SENSE2+ (Pins 2, 14): The (+) Input to each
Differential Current Comparator. The ITH pin voltage and
controlled offsets between the SENSE– and SENSE+ pins in
conjunction with R

set the current trip threshold.

SENSE

VIN = 15V
V

= 5V

OUT

LOAD STEP = 0A TO 3A
Burst Mode OPERATION

20µs/DIV

1876 G30

Constant Frequency (Burst
Inhibit) Operation (Figure 1)

V

OUT

20mV/DIV

I

OUT

0.5A/DIV

VIN = 15V

= 5V

V

OUT

= 5V

V

FCB

I

= 20mA

OUT

2µs/DIV

1876 G33

SENSE1–, SENSE2– (Pins 3, 13): The (–) Input to the
Differential Current Comparators.

V

OSENSE1

, V

OSENSE2

(Pins 4, 12): Receives the remotely-

sensed feedback voltage for each controller from an external
resistive divider across the output.

FREQSET (Pin 5): Frequency Control Input to the Oscillator.
This pin can be left open, tied to ground, tied to INTVCC or
driven by an external voltage source. This pin can also be used
with an external phase detector to build a true phase-locked
loop.

STBYMD (Pin 6): Control pin that determines which circuitry
remains active when the controllers are shut down and/or

1876fa

8

UUU

PIN FUNCTIONS

LTC1876

provides a common control point to shut down both control­lers. See the Operation section for details.

FCB (Pin 7): Forced Continuous Control Input. This input acts
on both controllers and is normally used to regulate a
secondary winding. Pulling this pin below 0.8V will force
continuous synchronous operation on both controllers. Do
not leave this pin floating.

I

TH1, ITH2

Regulator Compensation Point. Each associated channel’s
current comparator trip point increases with this control
voltage.

SGND (Pin 9): Small signal ground common to both control­lers, must be routed separately from high current grounds to
the common (–) terminals of the C

3.3V

supplying up to 10mA DC with peak currents as high as
50mA.

AUXSGND (Pin 15): Small Signal Ground of the Auxiliary
Boost Regulator.

AUXVFB (Pin 16): Auxiliary Boost Regulator Feedback Volt­age. This pin receives the feedback voltage from an external
resistive divider across the auxiliary output.

AUXSW (Pins 17, 18): Switch Node Connections to Inductor
for the Auxiliary Regulator. Voltage swing at these pins are
from ground to (V
Minimize trace area at these pins to keep EMI down.

AUXPGND (Pins 19, 20): The Auxiliary Power Ground Pins.
Its gate drive currents are returned to these pin.

(Pins 8, 11): Error Amplifier Output and Switching

capacitors.

OUT

(Pin 10): Output of a linear regulator capable of

OUT

+ voltage across Shottky diode).

OUT

TG1, TG2 (Pins 35, 24): High Current Gate Drives for Top
N-Channel MOSFETs. These are the outputs of floating
drivers with a voltage swing equal to INTV
posed on the switch node voltage SW.

SW1, SW2 (Pins 34, 25): Switch Node Connections to
Inductors. Voltage swing at these pins is from a Schottky
diode (external) voltage drop below ground to VIN.

BOOST1, BOOST2 (Pins 33, 26): Bootstrapped Supplies to
the Top Side Floating Drivers. Capacitors are connected
between the boost and switch pins and Schottky diodes are
tied between the boost and INTV
boost pins is from INTV

BG1, BG2 (Pins 31, 27): High Current Gate Drives for Bottom
(synchronous) N-Channel MOSFETs. Voltage swing at these
pins is from ground to INTVCC.

PGND (Pin 28): Driver Power Ground. Connects to sources
of bottom (synchronous) N-channel MOSFETs, anode of the
Schottky rectifier and the (–) terminal(s) of CIN.

INTVCC (Pin 29): Output of the Internal 5V Linear Low
Dropout Regulator and the EXTVCC Switch. The driver and
control circuits are powered from this voltage source. Must
be decoupled to power ground with a minimum of 4.7µF
tantalum or other, low ESR capacitor. The INTVCC regulator
standby operation is determined by the STBYMD pin.

EXTVCC (Pin 30): External Power Input to an Internal Switch
Connected to INTVCC. This switch closes and supplies V
power, bypassing the internal low dropout regulator, when­ever EXTVCC is higher than 4.7V. See EXTVCC connection in
Applications section. Do not exceed 7V on this pin.

to (VIN + INTVCC).

CC

pins. Voltage swing at the

CC

– 0.5V superim-

CC

CC

AUXVIN (Pin 21): Auxiliary Boost Regulator Controller Sup­ply Pin. Must be closely decoupled to AUXPGND.

AUXSD (Pin 22): Shutdown Pin for the Auxiliary Regulator.
Connect to 2.4V or more to enable the auxiliary regulator or
ground to shut the auxiliary regulator off.

VIN (Pin 32): Main Supply Pin. A bypass capacitor should be
tied between this pin and the signal ground pin.

PGOOD (Pin 36): Open-Drain Logic Output. PGOOD is pulled
to ground when the voltage on either V

7.5% of its setpoint.

pin is not within

OSENSE

1876fa

9

LTC1876

UU

W

FUNCTIONAL DIAGRA

CLK1
CLK2

V

OSENSE1

V

OSENSE2

BINH

FCB

REF

DUPLICATE FOR SECOND
CONTROLLER CHANNEL

1.19V

OSCILLATOR

WINDOW

COMPARATOR

4.5V

0.8V

+
–

+

–

1M

–
+

+
–

V

5V
LDO
REG

INTERNAL

SUPPLY

FREQSET

PGOOD

V

SEC

0.18µA

R6

FCB

R5

3.3V

OUT

V

IN

V

IN

4.8V

EXTV

CC

INTV

CC

5V

+

SGND

STBYMD

SRQ

Q

0.55V

I1 I2

+
–

0.86V

4(VFB)

SLOPE

COMP

1.2µA

6V

DROP

OUT
DET

45k

+
–

+– –+

4(VFB)

SHDN

RST

BOT

RUN/SS1

B

3mV

TOP ON

FCB

SHDN

START

–
+

45k

RUN
SOFT

2.4V

OV

EA

SWITCH

LOGIC

–
+

+
–

INTV

BOOST

CC

INTV

30k

30k

CC

TG

SW

BG

PGND

SENSE

SENSE

V

OSENSE

I

TH

RUN/SS

+

–

TOP

BOT

INTV

V

FB

0.80V

0.86V

V

CC

IN

D

B

C

B

D

SEC

R2

R1

C

C

C

C2

+

C

1

SENSE

IN

C

OUT

+

V

OUT

+

C

SEC

D

R

R

C

BOOST
REGULATOR

1.26V
V

REF

AUXV

FB

R8

C

SS

AUXSD

EA

AUX

+

–

R7

C

C

R

C

Σ

RAMP

GENERATOR

AUXV

IN

L3

D5

AUXV

A1

AUX

–
+

Q

R

S

Q

1.2MHz

OSCILLATOR

OSC

AUX

AUXSW

+
–

AUXPGND

OUT

+

C

OUTAUX

1876 FD/F02

Figure 2

1876fa

10

OPERATIO

LTC1876

U

(Refer to Functional Diagram)

Main Control Loop

The LTC1876 uses a constant frequency, current mode
scheme to provide excellent line and load regulation for all
its outputs. The step-down controllers have two of its
switch drivers operating at 180 degrees out of phase from
each other. During normal operation, each top MOSFET is
turned on when the clock for that channel sets the RS latch,
and turned off when the main current comparator, I1,
resets the RS latch. The peak inductor current at which I1
resets the RS latch is controlled by the voltage on the I
pin, which is the output of each error amplifier EA. The
V

OSENSE

compared to the internal reference voltage by the EA.
When the load current increases, it causes a slight de­crease in V
turn causes the ITH voltage to increase until the average
inductor current matches the new load current. After the
top MOSFET has turned off, the bottom MOSFET is turned
on until either the inductor current starts to reverse, as
indicated by current comparator I2, or the beginning of the
next cycle.

pin receives the voltage feedback signal, which is

OSENSE

relative to the 0.8V reference, which in

TH

AUX Regulator

The auxiliary boost regulator is completely independent
from other LTC1876 circuits. It can be operated even
though the LTC1876 step-down controllers are in shut­down. The operation of the boost regulator is similar to the
controllers. The oscillator, OSC
turns on the monolithic power switch. A voltage propor­tional to the switch current is added to a stabilizing ramp
and the resulting sum is fed into the positive terminal of the
PWM comparator, A1
level at the negative input of A1
turning off the power switch. The level at the negative input
of A1
an amplified version of the difference between the feed­back voltage and the reference voltage. Hence the error
amplifier sets the correct peak current level to keep the
output in regulation. To protect the power switch from
excessive current, a 1A minimum limit is internally set.
When the switch reaches this limit, it will force the latch to
reset, turning it off. Applying a voltage less than 0.5V on
the shutdown pin will put the boost regulator in shutdown.

is set by the error amplifier EA

AUX

. When this voltage exceeds the

AUX

, sets the RS latch and

AUX

, the SR latch is reset,

AUX

and is simply

AUX

The top MOSFET drivers are biased from floating boot­strap capacitor CB, which normally is recharged during
each off cycle through an external diode when the top
MOSFET turns off. As VIN decreases to a voltage close to
V

, the loop may enter dropout and attempt to turn on

OUT

the top MOSFET continuously. The dropout detector de­tects this and forces the top MOSFET off for about 500ns
every tenth cycle to allow CB to recharge.

The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor CSS. When
CSS reaches 1.5V, the main control loop is enabled with the
ITH voltage clamped at approximately 30% of its maximum
value. As CSS continues to charge, the I
gradually released allowing normal, full-current opera­tion. When both RUN/SS1 and RUN/SS2 are low, all
LTC1876 controller functions are shut down, and the
STBYMD pin determines if the standby 5V and 3.3V
regulators are kept alive.

pin voltage is

TH

Low Current Operation

The FCB pin is a multifunction pin providing two functions:

1) to provide regulation for a secondary winding by
temporarily forcing continuous PWM operation on both
controllers; and 2) select between
current operation. When the FCB pin voltage is below 0.8V,
the controller forces continuous PWM current operation.
In this mode, the top and bottom MOSFETs are alternately
turned on to maintain the output voltage independent of
direction of inductor current. When the FCB pin is below
V
Burst Mode operation. Burst Mode operation sets a mini­mum output current level before turning off the top switch
and turns off the synchronous MOSFET(s) when the
inductor current goes negative. This combination of re­quirements will, at low currents, force the ITH pin below a
voltage threshold that will temporarily inhibit turn-on of
both output MOSFETs until the output voltage drops
slightly. There is 60mV of hysteresis in the burst compara­tor B tied to the ITH pin. This hysteresis produces output
signals to the MOSFETs that turn them on for several

␣ –␣ 2V but greater than 0.8V, the controller enters

INTVCC

two

modes of low

1876fa

11