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LTC3631

3631fb

APPLICATIONS INFORMATION

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,

switching is enabled when VIN surpasses the internal

undervoltage 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 (50mA to 225mA).

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 LTC3631 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 50mA

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 50mA

to 225mA, 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 ﬂoating (programmed to 225mA 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 percentage

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

characteristics 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.

2. I2R losses are calculated from the resistances of the

internal switches, RSW, and external inductor RL. When

switching, the average output current ﬂowing through

the inductor is “chopped” between the high side PMOS

switch and the low side NMOS switch. Thus, the series

resistance looking back into the switch pin is a function

of the top and bottom switch RDS(ON) values and the

duty cycle (DC = VOUT/VIN) as follows:

RSW = (RDS(ON)TOP)DC + (RDS(ON)BOT)(1 – DC)

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