LTC1622CS8 datasheet(12/16 Pages) LINER

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LTC1622

APPLICATIONS INFORMATION

down to 2.7V. Let’s assume that the MOSFET dissipation

is to be limited to PP = 250mW and its thermal resistance

is 50

°C/W. Hence the junction temperature at TA = 25°C

will be 37.5

°C and δp = 0.005 (37.5 – 25) = 0.0625. The

required RDS(ON) is then given by:

DC I

DS ON

OUT

()

≅

() +

()

011

Ω

The P-channel MOSFET requirement can be met by an

Si6433DQ.

The requirement for the Schottky diode is the most strin-

gent when VOUT = 0V, i.e., short circuit. With a 0.025Ω

RSENSE resistor, the short-circuit current through the

Schottky is 0.1/0.025 = 4A. An MBRS340T3 Schottky

diode is chosen. With 4A flowing through, the diode is

rated with a forward voltage of 0.4V. Therefore, the worst-

case power dissipated by the diode is 1.6W. The addition

of DFB (Figure 5) will reduce the diode dissipation to

approximately 0.8W.

The input capacitor requires an RMS current rating of at

least 0.75A at temperature, and COUT will require an ESR

of 0.1

Ω for optimum efficiency.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1622. These items are illustrated graphically in the

layout diagram in Figure 6. Check the following in your

layout:

1. Is the Schottky diode closely connected between ground

at (–) lead of CIN and drain of the external MOSFET?

2. Does the (+) plate of CIN connect to the sense resistor

as closely as possible? This capacitor provides AC

current to the MOSFET.

3. Is the input decoupling capacitor (0.1

µF) connected

closely between VIN (Pin 8) and ground (Pin 6)?

4. Connect the end of RSENSE as close to VIN (Pin 8) as

possible. The VIN pin is the SENSE + of the current

comparator.

5. Is the trace from the SENSE– (Pin 1) to the Sense

resistor kept short? Does the trace connect close to

RSENSE?

6. Keep the switching node, SW, away from sensitive

small signal nodes.

7. Does the VFB pin connect directly to the feedback

resistors? The resistive divider R1 and R2 must be

connected between the (+) plate of COUT and signal

ground. Optional capacitor C1 should be located as

close as possible to the LTC1622.

R1 and R2 should be located as close as possible to the

LTC1622. R2 should connect to the output as close to

the load as practicable.

Figure 6. LTC1622 Layout Diagram (See PC Board Layout Checklist)

SENSE–

ITH

VFB

VIN

PDRV

GND

SYNC/

MODE

RUN/

BOLD LINES INDICATE HIGH CURRENT PATHS

RSENSE

1622 F06

0.1

µF

CITH

RITH

CSS

QUIET SGND

LTC1622

CIN

COUT

VOUT

VIN

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