LTC1735C View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1735C

High Efficiency Synchronous Step-Down Switching Regulator

Linear Technology 

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LTC1735

APPLICATIO S I FOR ATIO

R1

27k

C1

39pF

C2

0.1µF

C3

R2 100pF

100k

C4

100pF

C5

47pF

C6

1000pF

R3

680k

R4

C7

100k

0.1µF

1

COSC

2

RUN/SS

16

TG

15

BOOST

3

ITH

14

SW

U1

4

LTC1735

13

FCB

VIN

5

SGND

12

INTVCC

6

VOSENSE

11

BG

7 SENSE–

10

PGND

C8

0.22µF

D1

CMDSH-3

C9 +

1µF

C10

4.7µF

10V

8 SENSE+

9

EXTVCC

5V (OPTIONAL)

M1

FDS6680A

C9, C19: TAIYO YUDEN JMK107BJ105

C10: KEMET T494A475M010AS

C12 TO C14: TAIYO YUDEN GMK325F106

C15 TO C18: PANASONIC EEFUE0G181R

D1: CENTRAL SEMI CMDSH-3

D2: MOTOROLA MBRS340

L1: PANASONIC ETQP6F1R0SA

M1 TO M3: FAIRCHILD FDS6680A

R5: IRC LRF2512-01-R003-J

U1: LINEAR TECHNOLOGY LTC1735CS

C12 TO C14

10µF

35V

VIN

7.5V TO

24V

GND

D2

MBRS340

M2, M3

FDS6680A

×2

L1

R5

1µH 0.003Ω

C11

330pF

VOUT

1.5V

15A

C19

R6

1µF

10k

+ C15 TO

R7

11.5k

C18

180µF

4V

GND

1735 F08

Figure 8. CPU-Core-Voltage Regulator with Active Voltage Positioning

voltage cannot exceed ±0.082V. At VOUT = 1.5V, the

maximum output voltage change controlled by the ITH pin

would be:

∆VOSENSE

=

Input

Offset

VREF

•

VOUT

= ± 0.03V • 1.5 = ±56mV

0.8V

With the optimum resistor values at the ITH pin, the output

voltage will swing from 1.55V at minimum load to 1.44V

at full load. At this output voltage, active voltage position-

ing provides an additional ±56mV to the allowable tran-

sient voltage on the output capacitors, a 68% improve-

ment over the ±82mV allowed without active voltage

positioning.

The next step is to calculate the scale factor for VITH, the

ITH pin voltage. The VITH scale factor reflects the ITH pin

voltage required for a given load current. VITH controls the

peak sense resistor voltage, which represents the DC

output current plus one half of the peak-to-peak inductor

current. The no load to full load VITH range is from 0.3V to

2.4V, which controls the sense resistor voltage from 0V to

the ∆VSENSE(MAX) voltage of 75mV. The calculated VITH

scale factor with a 0.003Ω sense resistor is:

VITH

Scale Factor

=

VITH

Range • Sense Resistor

∆VSENSE(MAX)

Value

= (2.4V – 0.3V) • 0.003 = 0.084V/A

0.075V

VITH at any load current is:

VITH

=

IOUTDC

+

∆IL

2



•

VITH

Scale Factor

+VITH Offset

1735fc

23

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