Datasheet

LTC3621/LTC3621-2
9
3621fb
For more information www.linear.com/LTC3621
operaTion
selected, the part will transition in and out of sleep mode
depending on the output load current. This significantly
reduces the quiescent current, thus prolonging the use
of the input supply.
V
IN
Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient voltage spikes, the LTC3621 constantly
monitors the V
IN
pin for an overvoltage condition. When
V
IN
rises above 19V, the regulator suspends operation by
shutting off both power MOSFETs. Once V
IN
drops below
18.7V, the regulator immediately resumes normal opera
-
tion. The regulator executes its soft-start function when
exiting an overvoltage condition.
Output Voltage Programming
For non-fixed output voltage parts, the output voltage is
set by external resistive divider according to the following
equation:
V
OUT
= 0.6V • 1+
R2
R1




The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 1.
I
RMS
≅I
OUT(MAX)
V
OUT
V
IN
V
IN
V
OUT
–1
This formula has a maximum at V
IN
= 2V
OUT
, where:
I
RMS
≅
I
OUT
2
This simple worst-case condition is commonly used for
design because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based on only 2000 hours of life
which makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design. For low input
voltage applications, sufficient bulk input capacitance is
needed to minimize transient effects during output load
changes.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
Low Supply Operation
The LTC3621 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below 2.7V. As the input voltage rises slightly above the
undervoltage threshold, the switcher will begin its basic
operation. However, the R
DS(ON)
of the top and bottom
switch will be slightly higher than that specified in the
electrical characteristics due to lack of gate drive. Refer
to graph of R
DS(ON)
versus V
IN
for more details.
Soft-Start
The LTC3621 has an internal 800µs soft-start ramp. During
start-up soft-start operation, the switcher will operate in
pulse-skipping mode.
applicaTions inForMaTion
V
OUT
R2
R1
3621 F01
C
FF
LTC3621
SGND
FB
Figure 1. Setting the Output Voltage
Input Capacitor (C
IN
) Selection
The input capacitance, C
IN
, is needed to filter the square
wave current at the drain of the top power MOSFET. To
prevent large voltage transients from occurring, a low
ESR input capacitor sized for the maximum RMS current
should be used. The maximum RMS current is given by: