LTC1772CS6 datasheet(10/16 Pages) LINER

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LTC1772

APPLICATIONS INFORMATION

In the application, a 0.03

Ω resistor is used. For the

inductor, the required value is:

kHz

MIN =

−

















42 25

550

0 0288

003

25 03

42 03

200

In the application, a 5.6

µHinductorisusedtoreduceripple

current.

For the selection of the external MOSFET, the RDS(ON)

must be guaranteed at 2.5V since the LTC1772 has to work

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 0.03Ω

RSENSE resistor, the short-circuit current through the

Schottky is 0.1/0.03 = 3.3A. An MBRS340T3 Schottky

diode is chosen. With 3.3A flowing through, the diode is

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

case power dissipated by the diode is 1.32W. The addition

of DFB1 and DFB2 (Figure 5) will reduce the diode dissipa-

tion to approximately 0.66W.

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.

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

will be reduced to approximately 50% of the maximum

output current.

Figure 5. Foldback Current Limiting

VFB

ITH/RUN

VOUT

LTC1772

1772 F05

DFB1

DFB2

Design Example

Assume the LTC1772 is used in a single Lithium-Ion

battery-powered cellular phone application. The VIN will be

operating from a maximum of 4.2V down to a minimum of

2.7V. Load current requirement is a maximum of 1.5A but

most of the time it will be on standby mode, requiring only

2mA. Efficiency at both low and high load current is

important. Output voltage is 2.5V.

Maximum

OUT

IN MIN

Duty Cycle

()

From Figure 2, SF = 57%.

SENSE

OUT

= ()( )( ) = ()( ) =

100

057

12 1 5

0 0317

Ω

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