LTC3632_15 Datasheet, PDF (14/22 Pages) Linear Technology – High Efficiency, High Voltage 20mA Synchronous Step-Down Converter

LTC3632
APPLICATIONS INFORMATION
The minimum value of these thresholds is limited to the
internal V_IN UVLO thresholds that are shown in the Electri-
cal Characteristics table. The current that flows through
this divider will directly add to the shutdown, sleep and
active current of the LTC3632, 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
undervoltage lockout feature. In this configuration, switch-
ing 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 (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:
t_SS
=
C_SS
•
0.8V
5µA
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
I_SET pin to ground. The peak current soft-start will ramp
from 10mA to the final peak current value determined by a
resistor from I_SET to ground. A 1µA current is sourced out
14
of the I_SET pin. With only a capacitor connected between
I_SET and ground, the peak current ramps linearly from
10mA to 50mA, 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 float-
ing (programmed to 50mA 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 percent-
age 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 charac-
teristics and the internal MOSFET gate charge currents.
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