LTC3611_15 Datasheet, PDF(15/26 Page) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

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LTC3611_15 Datasheet, PDF (15/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

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LTC3611

Applications Information

with changes in VIN. Tying the FCB pin below the 0.6V

threshold forces continuous synchronous operation, al-

lowing current to reverse at light loads and maintaining

high frequency operation.

In addition to providing a logic input to force continuous

operation, the FCB pin provides a means to maintain a

ï¬yback winding output when the primary is operating

in discontinuous mode. The secondary output VOUT2 is

normally set as shown in Figure 4 by the turns ratio N

of the transformer. However, if the controller goes into

discontinuous mode and halts switching due to a light

primary load current, then VOUT2 will droop. An external

resistor divider from VOUT2 to the FCB pin sets a minimum

voltage VOUT2(MIN) below which continuous operation is

forced until VOUT2 has risen above its minimum:

VOUT2(MIN)

0.6V

ââ

â â

Fault Conditions: Current Limit and Foldback

The LTC3611 has a current mode controller which inher-

ently limits the cycle-by-cycle inductor current not only

in steady state operation but also in transient. To further

limit current in the event of a short circuit to ground, the

LTC3611 includes foldback current limiting. If the output

falls by more than 25%, then the maximum sense voltage is

progressively lowered to about one sixth of its full value.

INTVCC Regulator and EXTVCC Connection

An internal P-channel low dropout regulator produces the

5V supply that powers the drivers and internal circuitry

within the LTC3611. The INTVCC pin can supply up to 50mA

RMS and must be bypassed to ground with a minimum of

4.7Î¼F tantalum or ceramic capacitor. Good bypassing is

necessary to supply the high transient currents required

by the MOSFET gate drivers.

VOUT2

CSEC +

1ÂµF

VOUT1

COUT

PGND

64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49

SGND

IN4148

1:N

VIN

CIN

1 PGND

2 PGND

3 PGND

4 SW

5 SW

6 SW

7 SW

8 SW

9 SW

10 SW

11 SW

12 PVIN

13 PVIN

14 PVIN

15 PVIN

16 PVIN

LTC3611

SGND 48

SGND 47

SGND 46

SGND 45

EXTVCC 44

VFB 43

SGND 42

ION 41

SGND 40

FCB 39

ITH 38

VRNG 37

PGOOD 36

VON 35

SGND 34

SGND 33

OPTIONAL EXTVCC

CONNECTION

5V < VOUT2 < 7V

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

SGND

3611 F04

Figure 4. Secondary Output Loop and EXTVCC Connection

3611fd

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