LTC3632 LINER [Linear Technology], LTC3632 Datasheet - Page 14

• Manufacturer Part Number: LTC3632
• Description: High Effi ciency, High Voltage 20mA Synchronous Step-Down Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3632
rating of 6V. To keep the voltage on the HYST pin from
exceeding 6V, the following relation should be satisfi ed:
The RUN pin may also be directly tied to the V
for applications that do not require the programmable
undervoltage lockout feature. In this confi guration, switch-
ing is enabled when V
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
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:
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
from 10mA to the fi nal peak current value determined by a
resistor from I
of the I
I
10mA to 50mA, and the peak current soft-start time can
be expressed as:
14
SET
SET
V
t
t
SS
SS ISET
IN(MAX)
pin to ground. The peak current soft-start will ramp
and ground, the peak current ramps linearly from
(
=
SET
C
SS
)
pin. With only a capacitor connected between
•
=
•
SET
C
R1+R2 +R3
. 0 8
5
ISET
μA
to ground. A 1μA current is sourced out
V
R3
IN
•
0 8
surpasses the internal undervoltage
1
.
μA
V
< 6V
SET
pin (10mA to 50mA).
IN
supply
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
to ground, the peak current offset ramps as a decaying
exponential with a time constant of R
case, the peak current soft-start time is approximately
3 • R
Unlike the SS pin, the I
during an abnormal event; however, if the I
ing (programmed to 50mA peak current), the SS and I
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
Effi ciency Considerations
The effi ciency 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 effi ciency and which change would produce
the most improvement. Effi ciency can be expressed as:
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
operating current dominates the effi ciency loss at very
low load currents whereas the I
effi ciency loss at medium to high load currents.
1. The V
Effi ciency = 100% – (L1 + L2 + L3 + ...)
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 V
ground. The resulting dQ/dt is the current out of V
that is typically larger than the DC bias current.
SS(ISET)
ISET
IN
• C
operating current comprises two components:
• (0.8V/6μA).
ISET
IN
operating current and I
.
SET
pin does not get pulled to ground
2
R loss dominates the
ISET
2
R losses. The V
• C
SET
ISET
pin is fl oat-
. For this
IN
3632fb
SET
SET
to
IN
IN