LTC1625IGN datasheet(18/24 Pages) LINER

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LTC1625

APPLICATIONS INFORMATION

CSS

0.1

µF

10k

CC1

470pF

CC2

220pF

Si4412DY

CIN

µF

35V

×2

VIN

12V TO 22V

VOUT

3.3V

Si4412DY

MBRS140T3

CMDSH-3

CVCC

4.7

µF

1625 F09

OPEN

INTVCC

VIN

EXTVCC

LTC1625

SYNC

VPROG

BOOST

INTVCC

RUN/SS

FCB

ITH

SGND

VOSENSE

PGND

0.1

µF

µH

COUT

100

µF

10V

0.065

Ω

×2

CIN: AVX TPSE226M035R0300

COUT: AVX TPSD107M010R0065

L1: SUMIDA CDRH125-150MC

Figure 9. 3.3V/2A Fixed Output at 225kHz

CIN is chosen for an RMS current rating of at least 1A at

temperature. COUT is chosen with an ESR of 0.033Ω for

low output ripple. The output ripple in continuous mode

will be highest at the maximum input voltage and is

approximately:

∆VO = (∆IL(MAX))(ESR) = (0.83A)(0.033Ω) = 27mV

The complete circuit is shown in Figure 9.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1625. These items are also illustrated graphically in

the layout diagram of Figure 10. Check the following in

your layout:

1) Connect the TK lead directly to the drain of the topside

MOSFET. Then connect the drain to the (+) plate of CIN.

This capacitor provides the AC current to the top

MOSFET.

2) The power ground pin connects directly to the source of

the bottom N-channel MOSFET. Then connect the source

to the anode of the Schottky diode and (–) plate of CIN,

which should have as short lead lengths as possible.

3) The LTC1625 signal ground pin must return to the (–)

plate of COUT. Connect the (–) plate of COUT to power

ground at the source of the bottom MOSFET

4) Keep the switch node SW away from sensitive small-

signal nodes. Ideally the switch node should be placed

on the opposite side of the power MOSFETs from the

LTC1625.

5) Connect the INTVCC decoupling capacitor CVCC closely

to the INTVCC pin and the power ground pin. This

capacitor carries the MOSFET gate drive current.

6) Does the VOSENSE pin connect directly to the (+) plate of

COUT? In adjustable applications, the resistive divider

(R1, R2) must be connected between the (+) plate of

COUT and signal ground. Place the divider near the

LTC1625 in order to keep the high impedance VOSENSE

node short.

7) For applications with multiple switching power con-

verters connected to the same VIN, ensure that the input

filter capacitance for the LTC1625 is not shared with the

other converters. AC input current from another con-

verter will cause substantial input voltage ripple that

may interfere with proper operation of the LTC1625. A

few inches of PC trace or wire (

≈100nH) between CIN

and VIN is sufficient to prevent sharing.

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