LTC3631_15 Datasheet, PDF (14/22 Pages) Linear Technology – High Efficiency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
Applications Information
The minimum value of these thresholds is limited to
the internal V_IN UVLO thresholds that are shown in the
Electrical Characteristics table. The current that flows
through this divider will directly add to the shutdown,
sleep and active current of the LTC3631, and care should
be taken to minimize the impact of this current on the
overall efficiency of the application circuit. Resistor values
in the megohm range may be required to keep the impact
on quiescent shutdown and sleep currents low. Be aware
that the HYST pin cannot be allowed to exceed its absolute
maximum rating of 6V. To keep the voltage on the HYST
pin from exceeding 6V, the following relation should be
satisfied:
V_IN(MAX)
•



R1+
R3
R2 +
R3



<
6V
The RUN pin may also be directly tied to the V_IN supply
for applications that do not require the programmable un-
dervoltage lockout feature. In this configuration, switching
is enabled when V_IN surpasses the internal undervoltage
lockout threshold.
Soft-Start
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_{S S(IS E T )}
=
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.
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).
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
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.
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
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 charac-
teristics and the internal MOSFET gate charge currents.
3631fe
14
For more information www.linear.com/LTC3631