LTC3631 Datasheet, PDF (14/22 Pages) Linear Technology – High Effi ciency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
APPLICATIONS INFORMATION
The RUN pin may also be directly tied to the V_IN supply
for applications that do not require the programmable
undervoltage lockout feature. In this configuration,
switching is enabled when V_IN 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 I_SET 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:
t_SS
=
C_SS
•
0.8V
5μA
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 I_SET pin
to ground. The peak current soft-start will ramp from 50mA
to the final peak current value determined by a resistor
from I_SET to ground. A 1μA current is sourced out of the
I_SET pin. With only a capacitor connected between I_SET
and ground, the peak current ramps linearly from 50mA
to 225mA, and the peak current soft-start time can be
expressed as:
t_SS(ISET)
=
C_ISET
•
0.8V
1μA
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 I_SET
to ground, the peak current offset ramps as a decaying
exponential with a time constant of R_ISET • C_ISET. For this
case, the peak current soft-start time is approximately
3 • R_ISET • C_ISET.
14
Unlike the SS pin, the I_SET pin does not get pulled to
ground during an abnormal event; however, if the I_SET
pin is floating (programmed to 225mA peak current),
the SS and I_SET 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 C_SS(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: V_IN operating current and I²R losses. The V_IN
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 V_IN 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
V_IN to ground. The resulting dQ/dt is the current out of
V_IN that is typically larger than the DC bias current.
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. 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 R_DS(ON) values and the
duty cycle (DC = V_OUT/V_IN) as follows:
R_SW = (R_DS(ON)TOP)DC + (R_DS(ON)BOT)(1 – DC)
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