LTC3630A Datasheet, PDF (15/24 Pages) Linear Technology – High Efficiency, 76V 500mA Synchronous Step-Down Converter

LTC3630A
Applications Information
V_OUT
R1
LTC3630A V_{FB 5V}
0.8V
4.2M
800k
SS
V_PRG1
V_PRG2
R2
3630a F07
Figure 7. Setting the Output Voltage with
External and Internal Resistors
RUN Pin and External Input Undervoltage Lockout
The RUN pin has two different threshold voltage levels.
Pulling the RUN pin below 0.7V puts the LTC3630A into a
low quiescent current shutdown mode (I_Q ~ 5µA). When
the RUN pin is greater than 1.21V, the controller is enabled.
Figure 8 shows examples of configurations for driving the
RUN pin from logic.
The RUN pin can alternatively be configured as a precise
undervoltage (UVLO) lockout on the V_IN supply with a
resistive divider from V_IN to ground. A simple resistive
divider can be used as shown in Figure 9 to meet specific
V_IN voltage requirements.
The current that flows through the R3-R4 divider will
directly add to the shutdown, sleep, and active current
of the LTC3630A, and care should be taken to minimize
the impact of this current on the overall efficiency of the
application circuit. To keep the variation of the rising V_IN
UVLO threshold to less than 5% due to the internal pull-
up circuitry, the following equations should be used to
calculate R3 and R4:
R3 ≤ ^{RisingVINUVLOThreshold}
40µA
R4 =
R3 • 1.21V
RisingV_INUVLOThreshold – 1.21V +R3 • 4µA
The falling UVLO threshold will be about 10% lower than
the rising V_IN UVLO threshold due to the 110mV hysteresis
of the RUN comparator.
For applications that do not require a precise UVLO, the
RUN pin can be left floating. In this configuration, the UVLO
threshold is limited to the internal V_IN UVLO thresholds as
shown in the Electrical Characteristics table.
Be aware that the RUN pin cannot be allowed to exceed
its absolute maximum rating of 6V. To keep the voltage
on the RUN pin from exceeding 6V, the following relation
should be satisfied:
V_IN(MAX) < 4.5 • Rising V_IN UVLO Threshold
To support a V_IN(MAX) greater than 4.5x the external UVLO
threshold, an external 4.7V Zener diode should be used
in parallel with R4. See Figure 11.
Soft-Start
SUPPLY
LTC3630A
RUN
LTC3630A
RUN
3630a F08
Figure 8. RUN Pin Interface to Logic
5V
LTC3630A
Soft-start is implemented by ramping the effective refer-
ence voltage from 0V to 0.8V. To increase the duration of
soft-start, place a capacitor from the SS pin to ground.
An internal 5µA pull-up current will charge this capacitor.
The value of the soft-start capacitor can be calculated by
the following equation:
C_SS
=
Soft-Start
Time
•
5µA
0.35V
V_IN
R3
2M
RUN
R4
SLEEP, ACTIVE: 2µA
SHUTDOWN: 0µA
3630a F09
Figure 9. Adjustable UV Lockout
The minimum soft-start time is limited to the internal soft-
start timer of 0.8ms. When the LTC3630A detects a fault
condition (input supply undervoltage or overtemperature)
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.
3630af
For more information www.linear.com/LTC3630A
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