LINEAR TECHNOLOGY LTC1778, LTC1778-1 Technical data

FEATURES

No R

DESCRIPTIO

SENSE

LTC1778/LTC1778-1

Wide Operating Range,

TM

Step-Down Controller

U

■

No Sense Resistor Required

■

True Current Mode Control

■

Optimized for High Step-Down Ratios

■

t

ON(MIN)

■

Extremely Fast Transient Response

■

Stable with Ceramic C

■

Dual N-Channel MOSFET Synchronous Drive

■

Power Good Output Voltage Monitor (LTC1778)

■

Adjustable On-Time (LTC1778-1)

■

Wide VIN Range: 4V to 36V

■

±1% 0.8V Voltage Reference

■

Adjustable Current Limit

■

Adjustable Switching Frequency

■

Programmable Soft-Start

■

Output Overvoltage Protection

■

Optional Short-Circuit Shutdown Timer

■

Micropower Shutdown: IQ < 30µA

■

Available in a 16-Pin Narrow SSOP Package

≤ 100ns

OUT

U

APPLICATIO S

■

Notebook and Palmtop Computers

■

Distributed Power Systems

The LTC®1778 is a synchronous step-down switching
regulator controller optimized for CPU power. The con­troller uses a valley current control architecture to deliver
very low duty cycles with excellent transient response
without requiring a sense resistor. Operating frequency is
selected by an external resistor and is compensated for
variations in VIN.

Discontinuous mode operation provides high efficiency
operation at light loads. A forced continuous control pin
reduces noise and RF interference, and can assist second­ary winding regulation by disabling discontinuous opera­tion when the main output is lightly loaded.

Fault protection is provided by internal foldback current
limiting, an output overvoltage comparator and optional
short-circuit shutdown timer. Soft-start capability for sup­ply sequencing is accomplished using an external timing
capacitor. The regulator current limit level is user program­mable. Wide supply range allows operation from 4V to 36V
at the input and from 0.8V up to (0.9)VIN at the output.

, LTC and LT are registered trademarks of Linear Technology Corporation.
No R
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 6100678, 6580258, 5847554, 6304066

is a trademark of Linear Technology Corporation.

SENSE

TYPICAL APPLICATIO

R

ON

1.4MΩ

I

BOOST

LTC1778

INTV

V

SW

BG

PGND

V

ON

IN

TG

CB 0.22µF

D

B

CMDSH-3

CC

+

C

4.7µF

FB

C

C

500pF

C

SS

0.1µF

RUN/SS

I

TH

R

C

20k

SGND

PGOOD

Figure 1. High Efficiency Step-Down Converter

VCC

U

M1
Si4884

M2
Si4874

L1

1.8µH

D1
B340A

+

R2

30.1k

R1
14k

1778 F01a

C

IN

10µF
50V
×3

C

OUT

180µF
4V
×2

V

IN

5V TO 28V

V

OUT

2.5V
10A

Efficiency vs Load Current

100

V

OUT

90

80

EFFICIENCY (%)

70

60

0.01

= 2.5V

0.1
LOAD CURRENT (A)

VIN = 5V

VIN = 25V

1

10

1778 F01b

1778fb

1

LTC1778/LTC1778-1

TOP VIEW

GN PACKAGE

16-LEAD PLASTIC SSOP

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

RUN/SS

V

ON

V

RNG

FCB

I

TH

SGND

I

ON

V

FB

BOOST

TG

SW

PGND

BG

INTV

CC

V

IN

EXTV

CC

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

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

Boosted Topside Driver Supply Voltage

(BOOST) ................................................... 42V to –0.3V

SW Voltage .................................................. 36V to –5V

EXTV

, (BOOST – SW), RUN/SS,

CC

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

FCB, V
I

TH

, V

ON

Voltages .......... INTVCC + 0.3V to –0.3V

RNG

, VFB Voltages...................................... 2.7V to –0.3V

UU

W

PACKAGE/ORDER I FOR ATIO

RUN/SS

1

2

PGOOD

3

V

RNG

4

FCB

5

I

TH

6

SGND

7

I

ON

8

V

FB

16-LEAD PLASTIC SSOP

T

= 125°C, θJA = 130°C/ W

JMAX

TOP VIEW

GN PACKAGE

BOOST

16

15

TG

14

SW

13

PGND

12

BG

11

INTV

CC

10

V

IN

9

EXTV

CC

ORDER PART

NUMBER

LTC1778EGN
LTC1778IGN

GN PART MARKING

1778
1778I

TG, BG, INTVCC, EXTVCC Peak Currents.................... 2A

TG, BG, INTVCC, EXTVCC RMS Currents .............. 50mA

Operating Ambient Temperature Range (Note 4)

LTC1778E ........................................... – 40°C to 85°C

LTC1778I.......................................... – 40°C to 125°C

Junction Temperature (Note 2)............................ 125°C

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

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

ORDER PART

NUMBER

LTC1778EGN-1

GN PART MARKING

17781

T

= 125°C, θJA = 130°C/ W

JMAX

Consult LTC Marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Main Control Loop

I

Q

V

FB

∆V

FB(LINEREG)

∆V

FB(LOADREG)

I

FB

g

m(EA)

V

FCB

I

FCB

t

ON

t

ON(MIN)

2

Input DC Supply Current
Normal 900 2000 µA
Shutdown Supply Current 15 30 µA

Feedback Reference Voltage ITH = 1.2V (Note 3) LTC1778E ● 0.792 0.800 0.808 V

= 1.2V (Note 3) LTC1778I ● 0.792 0.800 0.812 V

I

TH

Feedback Voltage Line Regulation VIN = 4V to 30V, ITH = 1.2V (Note 3) 0.002 %/V

Feedback Voltage Load Regulation ITH = 0.5V to 1.9V (Note 3) ● –0.05 –0.3 %

Feedback Input Current VFB = 0.8V –5 ±50 nA

Error Amplifier Transconductance ITH = 1.2V (Note 3) ● 1.4 1.7 2 mS

Forced Continuous Threshold ● 0.76 0.8 0.84 V

Forced Continuous Pin Current V

On-Time ION = 30µA, VON = 0V (LTC1778-1) 198 233 268 ns

Minimum On-Time ION = 180µA 50 100 ns

= 0.8V –1 –2 µA

FCB

I

= 15µA, VON = 0V (LTC1778-1) 396 466 536 ns

ON

1778fb

LTC1778/LTC1778-1

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

t

OFF(MIN)

V

SENSE(MAX)

V

SENSE(MIN)

∆V

FB(OV)

V

FB(UV)

V

RUN/SS(ON)

V

RUN/SS(LE)

V

RUN/SS(LT)

I

RUN/SS(C)

I

RUN/SS(D)

V

IN(UVLO)

V

IN(UVLOR)

TG R

UP

TG R

DOWN

BG R

UP

BG R

DOWN

TG t

r

TG t

f

BG t

r

BG t

f

Internal VCC Regulator

V

INTVCC

∆V

LDO(LOADREG)

V

EXTVCC

∆V

EXTVCC

∆V

EXTVCC(HYS)

PGOOD Output (LTC1778 Only)

∆V

FBH

∆V

FBL

∆V

FB(HYS)

V

PGL

Minimum Off-Time ION = 30µA 250 400 ns

Maximum Current Sense Threshold V

– V

V

PGND

SW

Minimum Current Sense Threshold V

– V

V

PGND

SW

= 1V, VFB = 0.76V ● 113 133 153 mV

RNG

V

= 0V, VFB = 0.76V ● 79 93 107 mV

RNG

V

= INTVCC, VFB = 0.76V ● 158 186 214 mV

RNG

= 1V, VFB = 0.84V – 67 mV

RNG

V

= 0V, VFB = 0.84V – 47 mV

RNG

= INTVCC, VFB = 0.84V – 93 mV

V

RNG

Output Overvoltage Fault Threshold 5.5 7.5 9.5 %

Output Undervoltage Fault Threshold 520 600 680 mV

RUN Pin Start Threshold ● 0.8 1.5 2 V

RUN Pin Latchoff Enable Threshold RUN/SS Pin Rising 4 4.5 V

RUN Pin Latchoff Threshold RUN/SS Pin Falling 3.5 4.2 V

Soft-Start Charge Current V

Soft-Start Discharge Current V

= 0V –0.5 –1.2 –3 µA

RUN/SS

= 4.5V, VFB = 0V 0.8 1.8 3 µA

RUN/SS

Undervoltage Lockout VIN Falling ● 3.4 3.9 V

Undervoltage Lockout Release VIN Rising ● 3.5 4 V

TG Driver Pull-Up On Resistance TG High 2 3 Ω

TG Driver Pull-Down On Resistance TG Low 2 3 Ω

BG Driver Pull-Up On Resistance BG High 3 4 Ω

BG Driver Pull-Down On Resistance BG Low 1 2 Ω

TG Rise Time C

TG Fall Time C

BG Rise Time C

BG Fall Time C

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

Internal VCC Load Regulation ICC = 0mA to 20mA, V

EXTVCC Switchover Voltage ICC = 20mA, V

EXTVCC Switch Drop Voltage ICC = 20mA, V

= 3300pF 20 ns

LOAD

= 3300pF 20 ns

LOAD

= 3300pF 20 ns

LOAD

= 3300pF 20 ns

LOAD

= 4V ● 4.7 5 5.3 V

EXTVCC

= 4V –0.1 ±2%

EXTVCC

Rising ● 4.5 4.7 V

EXTVCC

= 5V 150 300 mV

EXTVCC

EXTVCC Switchover Hysteresis 200 mV

PGOOD Upper Threshold VFB Rising 5.5 7.5 9.5 %

PGOOD Lower Threshold VFB Falling –5.5 –7.5 –9.5 %

PGOOD Hysteresis VFB Returning 1 2 %

PGOOD Low Voltage I

= 5mA 0.15 0.4 V

PGOOD

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

Note 2: T
dissipation P

is calculated from the ambient temperature TA and power

J

as follows:

D

LTC1778E: TJ = TA + (PD • 130°C/W)

Note 3: The LTC1778 is tested in a feedback loop that adjusts V
a specified error amplifier output voltage (I

).

TH

to achieve

FB

Note 4: The LTC1778E 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. The LTC1778I is guaranteed over the full – 40°C to 125°C
operating temperature range.

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3

LTC1778/LTC1778-1

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Transient Response

Transient Response

(Discontinuous Mode)

Start-Up

V

OUT

50mV/DIV

I

L

5A/DIV

LOAD STEP 0A TO 10A

= 15V

V

IN

V

= 2.5V

OUT

FCB = 0V
FIGURE 9 CIRCUIT

Efficiency vs Load Current

100

DISCONTINUOUS

90

80

70

EFFICIENCY (%)

60

50

0.001

MODE

0.01
LOAD CURRENT (A)

20µs/DIV

= 10V

V

IN

V

OUT

EXTV
FIGURE 9 CIRCUIT

0.1

CONTINUOUS
MODE

= 2.5V

= 5V

CC

1

1778 G03

1778 G01

10

V

OUT

50mV/DIV

I

L

5A/DIV

LOAD STEP 1A TO 10A

= 15V

V

IN

V

= 2.5V

OUT

FCB = INTV

CC

FIGURE 9 CIRCUIT

Efficiency vs Input Voltage

100

FCB = 5V
FIGURE 9 CIRCUIT

95

I

LOAD

90

I

= 10A

LOAD

EFFICIENCY (%)

85

80

0

5101520

INPUT VOLTAGE (V)

20µs/DIV

= 1A

1778 G02

25 30

1778 G04

RUN/SS

2V/DIV

V

OUT

1V/DIV

I

L

5A/DIV

VIN = 15V

= 2.5V

V

OUT

R

= 0.25Ω

LOAD

Frequency vs Input Voltage

300

FCB = 0V
FIGURE 9 CIRCUIT

280

260

240

FREQUENCY (kHz)

220

200

5

I

10

INPUT VOLTAGE (V)

50ms/DIV

= 10A

OUT

I

= 0A

OUT

15

1778 G19

20

25

1778 G05

Frequency vs Load Current

300

CONTINUOUS MODE

250

200

150

100

FREQUENCY (kHz)

50

0

0

DISCONTINUOUS
MODE

2468

LOAD CURRENT (A)

4

1778 G26

Load Regulation

0

–0.1

(%)

–0.2

OUT

∆V

–0.3

10

–0.4

2

0

LOAD CURRENT (A)

4

FIGURE 9 CIRCUIT

6

8

1778 G06

10

ITH Voltage vs Load Current

2.5
FIGURE 9 CIRCUIT

2.0

1.5

CONTINUOUS

VOLTAGE (V)

1.0

TH

I

0.5

0

0

MODE

DISCONTINUOUS
MODE

5

LOAD CURRENT (A)

10

15

1778 G07

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UW

TEMPERATURE (°C)

–50

0.78

FEEDBACK REFERENCE VOLTAGE (V)

0.79

0.80

0.81

0.82

–25 0 25 50

1778 G12

75 100 125

TYPICAL PERFOR A CE CHARACTERISTICS

Current Sense Threshold
vs ITH Voltage

300

RNG

2V

=

V

On-Time vs ION Current

10k

V

VON

= 0V

LTC1778/LTC1778-1

On-Time vs VON Voltage

1000

I

ION

= 30µA

200

100

0

–100

CURRENT SENSE THRESHOLD (mV)

–200

0

1.0 1.5 2.0

0.5
ITH VOLTAGE (V)

On-Time vs Temperature

300

I

= 30µA

ION

= 0V

V

VON

250

200

150

ON-TIME (ns)

100

50

1.4V

1V

0.7V

0.5V

2.5 3.0

1778 G08

1k

ON-TIME (ns)

100

10

1

ION CURRENT (µA)

Current Limit Foldback

150

125

100

= 1V

V

RNG

75

50

25

10 100

1778 G20

800

600

400

ON-TIME (ns)

200

0

0

1

VON VOLTAGE (V)

Maximum Current Sense
Threshold vs V

300

250

200

150

100

50

RNG

2

Voltage

3

1778 G21

0

–50

–25 0

Maximum Current Sense
Threshold vs RUN/SS Voltage

150

125

100

MAXIMUM CURRENT SENSE THRESHOLD (mV)

= 1V

V

RNG

75

50

25

0

1.5

2 2.5 3 3.5

RUN/SS VOLTAGE (V)

50 100 125

25 75

TEMPERATURE (°C)

1778 G22

1778 G23

MAXIMUM CURRENT SENSE THRESHOLD (mV)

0

0

0.2 0.4 0.6 0.8
VFB (V)

Maximum Current Sense
Threshold vs Temperature

150

V

= 1V

RNG

140

130

120

110

MAXIMUM CURRENT SENSE THRESHOLD (mV)

100

–50 –25

0

TEMPERATURE (°C)

50

25

75

1778 G09

100

1778 G11

125

MAXIMUM CURRENT SENSE THRESHOLD (mV)

0

0.5

0.75

1.0 1.25 1.5
V

VOLTAGE (V)

RNG

1.75 2.0

Feedback Reference Voltage
vs Temperature

1778 G10

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5

LTC1778/LTC1778-1

TEMPERATURE (C)

–50

2.0

UNDERVOLTAGE LOCKOUT THRESHOLD (V)

2.5

3.0

3.5

4.0

–25 0 25 50

1778 G18

75 100 125

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Input and Shutdown Currents

Error Amplifier g

2.0

1.8

vs Temperature

m

vs Input Voltage

1200

1000

EXTVCC OPEN

INTV

Load Regulation

CC

60

50

SHUTDOWN CURRENT (µA)

0

–0.1

1.6

(mS)

m

g

1.4

1.2

1.0
–50 –25

25

0

TEMPERATURE (°C)

EXTVCC Switch Resistance
vs Temperature

10

8

6

4

SWITCH RESISTANCE (Ω)

CC

2

EXTV

800

600

400

INPUT CURRENT (µA)

200

50

75

100

125

1778 G13

0

0

510

SHUTDOWN

EXTVCC = 5V

20 30 35

15 25

INPUT VOLTAGE (V)

1778 G24

40

30

20

10

0

–0.2

(%)

CC

–0.3

∆INTV

–0.4

–0.5

10

0

INTVCC LOAD CURRENT (mA)

30

40

20

50

1778 G25

RUN/SS Pin Current

FCB Pin Current vs Temperature

0

–0.25

–0.50

–0.75

–1.00

FCB PIN CURRENT (µA)

–1.25

vs Temperature

3

2

1

0

FCB PIN CURRENT (µA)

–1

PULL-DOWN CURRENT

PULL-UP CURRENT

0

–50 –25

6

25

0

TEMPERATURE (°C)

RUN/SS THRESHOLD (V)

50

75

100

125

1778 G14

RUN/SS Latchoff Thresholds
vs Temperature

5.0

4.5

LATCHOFF ENABLE

4.0

3.5

3.0
–50

LATCHOFF THRESHOLD

–25 0 25 50

TEMPERATURE (°C)

–1.50

–50

–25 0

75 100 125

1778 G17

50 100 125

25 75

TEMPERATURE (°C)

1778 G15

–2

–50 –25

25

0

TEMPERATURE (°C)

Undervoltage Lockout Threshold
vs Temperature

50

75

100

125

1778 G16

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LTC1778/LTC1778-1

U

UU

PI FU CTIO S

RUN/SS (Pin 1): Run Control and Soft-Start Input. A
capacitor to ground at this pin sets the ramp time to full
output current (approximately 3s/µF) and the time delay
for overcurrent latchoff (see Applications Information).
Forcing this pin below 0.8V shuts down the device.

PGOOD (Pin 2, LTC1778): Power Good Output. Open
drain logic output that is pulled to ground when the output
voltage is not within ±7.5% of the regulation point.

(Pin 2, LTC1778-1): On-Time Voltage Input. Voltage

V

ON

trip point for the on-time comparator. Tying this pin to the
output voltage or an external resistive divider from the
output makes the on-time proportional to V
comparator input defaults to 0.7V when the pin is grounded
or unavailable (LTC1778) and defaults to 2.4V when the
pin is tied to INTVCC. Tie this pin to INTVCC in high V
applications to use a lower RON value.

V

(Pin 3): Sense Voltage Range Input. The voltage at

RNG

this pin is ten times the nominal sense voltage at maxi­mum output current and can be set from 0.5V to 2V by a
resistive divider from INTVCC. The nominal sense voltage
defaults to 70mV when this pin is tied to ground, 140mV
when tied to INTVCC.

FCB (Pin 4): Forced Continuous Input. Tie this pin to
ground to force continuous synchronous operation at low
load, to INTVCC to enable discontinuous mode operation
at low load or to a resistive divider from a secondary output
when using a secondary winding.

I

(Pin 5): Current Control Threshold and Error Amplifier

TH

Compensation Point. The current comparator threshold
increases with this control voltage. The voltage ranges
from 0V to 2.4V with 0.8V corresponding to zero sense
voltage (zero current).

SGND (Pin 6): Signal Ground. All small-signal compo­nents and compensation components should connect to
this ground, which in turn connects to PGND at one point.

OUT

. The

OUT

ION (Pin 7): On-Time Current Input. Tie a resistor from V
to this pin to set the one-shot timer current and thereby set
the switching frequency.

VFB (Pin 8): Error Amplifier Feedback Input. This pin
connects the error amplifier input to an external resistive
divider from V

EXTVCC (Pin 9): External VCC Input. When EXTVCC ex-
ceeds 4.7V, an internal switch connects this pin to INTV
and shuts down the internal regulator so that controller
and gate drive power is drawn from EXTVCC. Do not exceed
7V at this pin and ensure that EXTVCC < VIN.

VIN (Pin 10): Main Input Supply. Decouple this pin to
PGND with an RC filter (1Ω, 0.1µF).

INTVCC (Pin 11): Internal 5V Regulator Output. The driver
and control circuits are powered from this voltage. De­couple this pin to power ground with a minimum of 4.7µF
low ESR tantalum capacitor.

BG (Pin 12): Bottom Gate Drive. Drives the gate of the
bottom N-channel MOSFET between ground and INTVCC.

PGND (Pin 13): Power Ground. Connect this pin closely to
the source of the bottom N-channel MOSFET, the (–)
terminal of C

SW (Pin 14): Switch Node. The (–) terminal of the boot­strap capacitor CB connects here. This pin swings from a
diode voltage drop below ground up to VIN.

TG (Pin 15): Top Gate Drive. Drives the top N-channel
MOSFET with a voltage swing equal to INTVCC superim­posed on the switch node voltage SW.

BOOST (Pin 16): Boosted Floating Driver Supply. The (+)
terminal of the bootstrap capacitor CB connects here. This
pin swings from a diode voltage drop below INTVCC up to
V

+ INTVCC.

IN

.

OUT

and the (–) terminal of CIN.

VCC

IN

CC

1778fb

7

LTC1778/LTC1778-1

U

U

W

FU CTIO AL DIAGRA

R

ON

**

I

7

ON

R

SQ

20k

+

–

+

I

REV

–

×

0.7V22.4V

1

tON = (10pF)

1.4V

V

RNG

3

0.7V

V

V

I

CMP

ON

I

VON
ION

1µA

V

IN

V

0.8V
REF

5V

REG

10

BOOST

16

TG

15

SW

14

INTV

CC

11

BG

12

PGND

13

PGOOD*

2

IN

+

C

IN

C

B

M1

L1

D

B

V

OUT

+

C

C

VCC

M2

OUT

FCB

4

–

F

0.8V

+

4.7V

+

–

SHDN

FCNT

ON

OV

9

SWITCH

EXTV

LOGIC

CC

1

240k

I

+
–

×4

LTC1778

*

LTC1778-1

**

THB

0.8V

3.3µA

0.74V

Q2

Q4

Q6

Q3

Q1

EA

–

+

0.8V

1V

Q5

0.6V

RUN

SHDN

1

RUN/SS

SS

–

+

+

–

0.6V

C

I

5

C1

TH

R

C

UV

OV

1.2µA

6V

+

–

+

0.86V

–

C

SS

V

FB

8

SGND

6

1778 FD

R2

R1

8

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