LINEAR TECHNOLOGY LTC3770 Technical data

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High Accuracy Synchronous Step-Down Controller Provides
Output Tracking and Programmable Margining –

Design Note 364

Charlie Zhao

Introduction

Output voltage tracking and margining are two increas­ingly popular features in power supply designs for high
performance server, ASIC and memory systems. These
two functions are among the advanced features provided
in the LTC

®

3770, a wide operating range, high accuracy,
synchronous step-down DC/DC controller. The LTC3770
operates with input voltages from 4V to 32V, generating
output voltages down to 0.6V with its highly accurate
±0.67% 0.6V reference voltage. It has a constant on-time,
valley current mode control architecture, which allows the
LTC3770 to operate at very low duty cycles with fast
transient response. The current sensing resistor is op­tional: leave it out for high efficiency, or in for most
accurate current limit.

MARGIN1

MARGIN0

TRACK

INTV

CC

R1
39k

R2
11k

C

1000pF

PGOOD

INTV

R

PG

V

OUT

C5
220pF

R3

95.3k
1%

R4

30.1k
1%

R

C

10k
1%

C1

CMPZ4683

C5
100pF

R
133k
1%

D2

3V

ON

100k

PGOOD

1

V

RNG

2

V

FB

3

I

TH

4

SGND

5

MARGIN1

6

MARGIN0

7

I

ON

8

V

REFIN

R5

V

REFOUT

10k

91011 1213141516

R6 82k

RUN INTV

V

IN

CC

R

RUN

100k

V

OUT

32 31 30 29 28 27 26 25

RUN

FCB

V

ON

MPGM TRACK/SS

C

0.01µF

Z0 BOOST TG SW

LTC3770EUH

PLLFLTR PLLIN VINV

R

PL

10k

C

PL

1000pF

Figure 1 shows a 2.5V, 10A power supply with a 5V to 28V
input range. The switching frequency of the converter can
be selected by an external resistor, R
sated for variations in input supply voltage, or the control­ler can be synchronized to an external clock via an internal
phase-lock loop. Figure 2 shows the efficiency of the
circuit versus load current.

Start-Up and Shut Down Output Tracking

The LTC3770 output voltage can track another supply via
the TRACK/SS pin. Tracking and sequencing functions
allow the user to easily optimize the start-up and shut
down of multiple supplies, such as system core and I/O

, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.

CC

R7

23.2k
1%

R8

26.7k
1%

C1

D

B

C
10µF

6.3V

0.22µF

VCC

CMDSH-3-LTC

24

SENSE+

23

SENSE–

22

PGND

21

BG

INTV

20

DRV

CC

INTV

CC

Z2

Z1

INSNSZVIN

INTV

P

C

VIN

0.1µF

CC

CC

19

18

17

RV

IN

10Ω

V

IN

, and is compen-

ON

C

10µF

50V

L1

1.8µH

BI TECH

HM65-H1R8-TB

D1
B340LA

C2

22µF

6.3V

×3

M1
PH3230

M2
PH3230

V

IN

5V TO 28V

IN

V

OUT

2.5V
10A

+

C4
180µF
4V

DN364 F01

05/05/364

Figure 1. High Efficiency 5V-28VIN to 2.5V/10A Synchronous Buck Converter with Tracking and Margining

100

98

96

94

92

90

88

EFFICIENCY (%)

86

84

82

80

0

2

VIN = 5V

VIN = 12V

6

4

LOAD CURRENT (A)

8

10

DN364 F02

Figure 2. Efficiency Curves of the
LTC3770 Converter in Figure 1

supplies. The tracking can be coincident or ratiometric, as
shown in Figure 3.

Programmable Voltage Margining

Voltage margining is the dynamic adjustment of the
output voltage to its worst-case operating value during
production test in order to stress the load circuitry, verify
control/protection functionality of the board and confirm
system reliability. The LTC3770 has two logic control
pins, MARGIN1 and MARGIN0, to enable margin up for
higher output voltage or margin down for lower output
voltage. Table 1 shows a summary of the configurations.

Table 1. Margining Function

MARGIN1 MARGIN0 MODE

Low Low No Margining

Low High Margin Up

High Low Margin Down

High High No Margining

The magnitude of the margin voltage shift is programmed
by selecting the ratio of two resistors, R5 and R6 in
Figure 1. When the margining function is enabled, the
error amplifier reference voltage is adjusted to:

R

VVV

=±

REFIN

⎛

06 118

..•

⎜
⎝

5

⎞
⎟

⎠

R

6

For example, ±5% margining can be achieved by select­ing R5 = 13k and R6 = 510k.

EXTERNAL

VOLTAGE

1V/DIV

V

OUT

1V/DIV

20ms/DIV

DN364 F03a

(3a) Coincident Tracking

EXTERNAL

VOLTAGE

1V/DIV

V

OUT

1V/DIV

20ms/DIV

DN364 F03b

(3b) Ratiometric Tracking

EXTERNAL

VOLTAGE

1V/DIV

V

OUT

1V/DIV

20ms/DIV

DN364 F03c

(3c) Special Ratiometric Tracking

Figure 3. Up/Down Output Tracking. Upper Waveform:
External Voltage; Lower Waveform: Output Voltage of the
LTC3770 Converter

Additional Features

The LTC3770 provides very strong gate drivers allowing
up to 25A output currents, programmable current limit,
output overvoltage protection and input undervoltage
lockout. Other features include power good monitor,
programmable soft-start, selectable discontinuous op­eration mode or forced continuous mode at light load and
adjustable dead time between the top gate and bottom
gate signals to optimize efficiency.

Conclusion

The LTC3770 has an accurate 0.6V reference voltage and
a wide operating range. It includes advanced functions
usually implemented by additional ICs—such as output
tracking and programmable voltage margining. Available
package options are a thermally enhanced 5mm × 5mm
QFN package and a leaded 28-pin SSOP.

Data Sheet Download

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

For applications help,

call [phone], Ext. 2593

dn364f LT/TP 0505 409K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2005