LTC3632 datasheet(14/22 Pages) LINER

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LTC3632

3632fb

APPLICATIONS INFORMATION

rating of 6V. To keep the voltage on the HYST pin from

exceeding 6V, the following relation should be satisﬁed:

VIN(MAX) •

R1+R2 +R3

< 6V

The RUN pin may also be directly tied to the VIN supply

for applications that do not require the programmable

undervoltage lockout feature. In this conﬁguration, switch-

ing is enabled when VINsurpassestheinternalundervoltage

lockout threshold.

Soft-Start

The internal 0.75ms soft-start is implemented by ramping

both the effective reference voltage from 0V to 0.8V and the

peak current limit set by the ISET pin (10mA to 50mA).

To increase the duration of the reference voltage soft-start,

place a capacitor from the SS pin to ground. An internal

5μA pull-up current will charge this capacitor, resulting in

a soft-start ramp time given by:

μA

•

When the LTC3632 detects a fault condition (input supply

undervoltage or overvoltage) or when the RUN pin falls

below 1.1V, the SS pin is quickly pulled to ground and the

internal soft-start timer is reset. This ensures an orderly

restart when using an external soft-start capacitor.

The duration of the 0.75ms internal peak current soft-

start may be increased by placing a capacitor from the

ISET pin to ground. The peak current soft-start will ramp

from 10mA to the ﬁnal peak current value determined by a

resistor from ISET to ground. A 1μA current is sourced out

of the ISET pin. With only a capacitor connected between

ISET and ground, the peak current ramps linearly from

10mA to 50mA, and the peak current soft-start time can

be expressed as:

μA

SS ISET

ISET

()

•

A linear ramp of peak current appears as a quadratic

waveform on the output voltage. For the case where the

peak current is reduced by placing a resistor from ISET

to ground, the peak current offset ramps as a decaying

exponential with a time constant of RISET • CISET. For this

case, the peak current soft-start time is approximately

3 • RISET • CISET.

Unlike the SS pin, the ISET pin does not get pulled to ground

during an abnormal event; however, if the ISET pin is ﬂoat-

ing (programmed to 50mA peak current), the SS and ISET

pins may be tied together and connected to a capacitor

to ground. For this special case, both the peak current

and the reference voltage will soft-start on power-up and

after fault conditions. The ramp time for this combination

is CSS(ISET) • (0.8V/6μA).

Efﬁciency Considerations

The 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 what is

limiting the efﬁciency and which change would produce

the most improvement. Efﬁciency can be expressed as:

Efﬁciency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of

the losses: VIN operating current and I2R losses. The VIN

operating current dominates the efﬁciency loss at very

low load currents whereas the I2R loss dominates the

efﬁciency loss at medium to high load currents.

1. The VIN operating current comprises two components:

The DC supply current as given in the electrical charac-

teristics and the internal MOSFET gate charge currents.

The gate charge current results from switching the gate

capacitance of the internal power MOSFET switches.

Each time the gate is switched from high to low to

high again, a packet of charge, dQ, moves from VIN to

ground. The resulting dQ/dt is the current out of VIN

that is typically larger than the DC bias current.

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