LTC3624_15 Datasheet, PDF (10/20 Pages) Linear Technology – 17V, 2A Synchronous Step-Down Regulator with 3.5A Quiescent Current

LTC3624/LTC3624-2
Operation
alive. However, if Burst Mode operation or pulse-skipping
mode is selected, the part will transition in and out of
sleep mode depending on the output load current. This
significantly reduces the quiescent current, thus prolong-
ing the use of the input supply.
V_IN Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient voltage spikes, the LTC3624/LTC3624-2
constantly monitors the V_IN pin for an overvoltage condi-
tion. When V_IN rises above 19V, the regulator suspends
operation by shutting off both power MOSFETs. Once V_IN
drops below 18.5V, the regulator immediately resumes
normal operation. The regulator executes its soft-start
function when exiting an overvoltage condition.
Minimum On-Time
The minimum on-time is the smallest duration of the time
the top power switch is allowed to be in its on state. This
time is typically 60ns. In forced continuous mode operation,
the minimum on-time limit imposes a minimum duty cycle
of 6% for the LTC3624 (F_SW = 1MHz) and 13.5% for the
LTC3624-2 (F_SW = 2.25MHz). In the rare cases that this
minimum on-time is violated, the output voltage may lose
regulation. In such situation, the user must choose either
Burst Mode or pulse-skipping mode operation, or apply a
slower external clock to force a slower switching frequency
in order to adhere to the minimum on-time limitation.
Low Supply Operation
The LTC3624 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 LTC3624/LTC3624-2 has an internal 1ms soft-start
ramp. During start-up soft-start operation, the switcher
will operate in pulse-skipping mode.
Applications Information
Output Voltage Programming
The output voltage is set by external resistive divider ac-
cording to the following equation for adjustable output
versions:
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.
For fixed V_OUT options, connect FB pin directly to V_OUT.
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
V_OUT
R2
C_FF
FB
LTC3624
R1
SGND
36242 F01
Figure 1. Setting the Output Voltage (Adjustable Version)
FB
LTC3624
(FIXED V_OUT)
SGND
V_OUT
36242 F02
Figure 2. Setting the Output Voltage (Fixed V_OUT Option)
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36242fc