LTC1624CS datasheet(10/28 Pages) LINER

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LTC1624

pin through a small Schottky diode (like a Central

CMDSH-3) to VOUT as shown in Figure 10. However, for

3.3V and other lower voltage regulators, additional cir-

cuitry is required to derive boost supply power from the

output.

For low input voltage operation (VIN < 7V), a Schottky

diode can be connected from VIN to BOOST to increase the

external MOSFET gate drive voltage. Be careful not to

exceed the maximum voltage on BOOST to SW pins

of 7.8V.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

OUT =+









119

The external resistive divider is connected to the output as

shown in Figure 2, allowing remote voltage sensing. When

using remote sensing, a local 100

Ω resistor should be

connected from L1 to R2 to prevent VOUT from running

away if the sense lead is disconnected.

APPLICATIONS INFORMATION

ITH/RUN

1624 F03

3.3V

OR 5V

ITH/RUN

(a)

(b)

ITH/RUN

(c)

Figure 3. ITH/ RUN Pin Interfacing

Soft start can be implemented by ramping the voltage on

ITH/RUN during start-up as shown in Figure 3(c). As the

voltage on ITH/RUN ramps from 1.19V to 2.4V the internal

peak current limit is also ramped at a proportional linear

rate. The peak current limit begins at approximately

10mV/RSENSE (at VITH/RUN = 1.4V) and ends at:

160mV/RSENSE (VITH/RUN = 2.4V)

The output current thus ramps up slowly, charging the

output capacitor. The peak inductor current and maximum

output current are as follows:

IL(PEAK) = (VITH/RUN – 1.3V)/(6.8RSENSE)

IOUT(MAX) = ILPEAK – ∆IL/2

with

∆IL = ripple current in the inductor.

During normal operation the voltage on the ITH/RUN pin

will vary from 1.19V to 2.4V depending on the load current.

Pulling the ITH/RUN pin below 0.8V puts the LTC1624 into

a low quiescent current shutdown (IQ< 30µA). This pin can

be driven directly from logic as shown in Figures 3(a)

and 3(b).

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

ITH/RUN Function

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

loop compensation and a means to shut down the LTC1624.

Soft start can also be implemented with this pin. Soft start

reduces surge currents from VIN by gradually increasing

the internal current limit.

Power supply sequencing can

also be accomplished using this pin.

An internal 2.5

µA current source charges up the external

capacitor CC. When the voltage on ITH/RUN reaches 0.8V

the LTC1624 begins operating. At this point the error

amplifier pulls up the ITH/RUN pin to its maximum of 2.4V

(assuming VOUT is starting low).

Figure 2. Setting the LTC1624 Output Voltage

LTC1624

VFB

GND

100pF

VOUT

1624 F02

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