LTC1772CS6 datasheet(10/12 Pages) LINER

10 / 12 page

LTC1772

1772fb

APPLICATIONS INFORMATION

0.4V, the loss increases from 0.5% to 8% as the load

current increases from 0.5A to 2A.

5. Transition losses apply to the external MOSFET and

increase at higher operating frequencies and input

voltages. Transition losses can be estimated from:

Transition Loss = 2(VIN)2IO(MAX)CRSS(f)

Other losses including CIN and COUT ESR dissipative

losses, and inductor core losses, generally account for

less than 2% total additional loss.

Foldback Current Limiting

As described in the Output Diode Selection, the worst-case

dissipation occurs with a short-circuited output when the

diode conducts the current limit value almost continu-

ously. To prevent excessive heating in the diode, foldback

current limiting can be added to reduce the current in

proportion to the severity of the fault.

Foldback current limiting is implemented by adding diodes

DFB1 and DFB2 between the output and the ITH/RUN pin as

shown in Figure 5. In a hard short (VOUT = 0V), the current

VFB

ITH/RUN

VOUT

LTC1772

1772 F05

DFB1

DFB2

Figure 6. LTC1772 Layout Diagram (See PC Board Layout Checklist)

BOLD LINES INDICATE HIGH CURRENT PATHS

RSENSE

VIN

VOUT

1772 F06

0.1

µF

COUT

CITH

RITH

CIN

ITH/RUN

LTC1772

GND

VFB

PGATE

VIN

SENSE–

Figure 5. Foldback Current Limiting

will be reduced to approximately 50% of the maximum

output current.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1772. 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

(Pin 2) 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 5) and ground (Pin 2)?

4. Connect the end of RSENSE as close to VIN (Pin 5) as

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

comparator.

5. Is the trace from SENSE– (Pin 4) to the Sense resistor

kept short? Does the trace connect close to RSENSE?

6. Keep the switching node PGATE 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.

LTC1772CS6 Datasheet(PDF) 10 Page - Linear Technology

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