LTC1435CS datasheet(12/20 Pages) LINER

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LTC1435

APPLICATIONS INFORMATION

EXTVCC

VIN

PGND

LTC1435

N-CH

CIN

VIN

COUT

RSENSE

VOUT

µF

LTC1435 • F04b

VN2222LL

BAT85

0.22

µF

Figure 4b. Capacitive Charge Pump for EXTVCC

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CB connected to the Boost

pin supplies the gate drive voltage for the topside MOSFET.

Capacitor CB in the Functional Diagram is charged through

diode DB from INTVCC when the SW pin is low. When the

topside MOSFET is to be turned on, the driver places the

CB voltage across the gate source of the MOSFET. This

enhances the MOSFET and turns on the topside switch.

The switch node voltage SW rises to VIN and the Boost pin

rises to VIN + INTVCC. The value of the boost capacitor CB

needs to be 100 times greater than the total input capaci-

tance of the topside MOSFET. In most applications 0.1

µF

is adequate. The reverse breakdown on DB must be greater

than VIN(MAX).

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

OUT =+









119

The external resistor divider is connected to the output as

shown in Figure 5 allowing remote voltage sensing.

Run/ Soft Start Function

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

soft start function and a means to shut down the LTC1435.

Figure 6. RUN/SS Pin Interfacing

LTC1435 • F06

CSS

3.3V OR 5V

RUN/SS

CSS

RUN/SS

Soft start reduces surge currents from VIN by gradually

increasing the internal current limit.

Power supply se-

quencing can also be accomplished using this pin.

An internal 3

µA current source charges up an external

capacitor CSS. When the voltage on RUN/SS reaches 1.3V

the LTC1435 begins operating. As the voltage on RUN/SS

continues to ramp from 1.3V to 2.4V, the internal current

limit is also ramped at a proportional linear rate. The

current limit begins at approximately 50mV/RSENSE (at

VRUN/SS = 1.3V) and ends at 150mV/RSENSE (VRUN/SS >

2.7V). The output current thus ramps up slowly, charging

the output capacitor. If RUN/SS has been pulled all the way

to ground there is a delay before starting of approximately

500ms/

µF, followed by an additional 500ms/µF to reach

full current.

tDELAY = 5(10

5)CSS Seconds

Pulling the RUN/SS pin below 1.3V puts the LTC1435 into

a low quiescent current shutdown (IQ < 25µA). This pin

can be driven directly from logic as shown in Figure 6.

Diode D1 in Figure 6 reduces the start delay but allows

CSS to ramp up slowly for the soft start function; this

diode and CSS can be deleted if soft start is not needed.

The RUN/SS pin has an internal 6V Zener clamp (See

Functional Diagram).

VOSENSE

LTC1435

LTC1435 • F05

100pF

1.19V

≤ VOUT ≤ 9V

SGND

Figure 5. Setting the LTC1435 Output Voltage

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