LTC1622CS8 datasheet(11/16 Pages) LINER

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LTC1622

APPLICATIONS INFORMATION

Transition Loss = 3(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.

Run/Soft-Start Function

The RUN/SS pin is a dual purpose pin that provides the

soft-start function and a means to shut down the LTC1622.

Soft-start reduces input surge current from VIN by gradu-

ally increasing the internal current limit. Power supply

sequencing can also be accomplished using this pin.

An internal 2.5

µA current source charges up an external

capacitor CSS. When the voltage on the RUN/SS reaches

0.7V the LTC1622 begins operating. As the voltage on

RUN/SS continues to ramp from 0.7V to 1.8V, the internal

current limit is also ramped at a proportional linear rate.

The current limit begins near 0A (at VRUN/SS = 0.7V) and

ends at 0.1/RSENSE (VRUN/SS ≥ 1.8V). The output current

thus ramps up slowly, reducing the starting surge current

required from the input power supply. If the RUN/SS has

been pulled all the way to ground, there will be a delay

before the current limit starts increasing and is given by:

tDELAY = 2.8 • 105 • CSS in seconds

Pulling the RUN/SS pin below 0.4V puts the LTC1622 into

a low quiescent current shutdown (IQ < 15µA).

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

continuously. 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 diode

DFB (1N4148 or equivalent) between the output and the ITH

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.

VFB

ITH

VOUT

LTC1622

1622 F05

DFB

Figure 5. Foldback Current Limiting

Design Example

Assume the LTC1622 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.

In the above application, Burst Mode operation is enabled

by connecting Pin 5 to VIN.

Maximum

OUT

IN MIN

Duty Cycle

()

From Figure 2, SF = 57%.

Use the curve of Figure 2 since the operating frequency is

the free running frequency of the LTC1622.

SENSE

OUT

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

100

057

15 1 5

0 0253

Ω

In the application, a 0.025

Ω resistor is used. For the

inductor, the required value is:

kHz

MIN =

−

















42 25

550

0 036

0 025

25 03

42 03

133

.µ

In the application, a 3.9

µH inductor is used to reduce

inductor ripple current and thus, output voltage ripple.

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

must be guaranteed at 2.5V since the LTC1622 has to work

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