LTC3630_15 Datasheet, PDF (16/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous Step-Down Converter

LTC3630
Applications Information
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.
5V
V_IN
R3
2M
RUN
LTC3630
SLEEP, ACTIVE: 2µA
SHUTDOWN: 0µA
Soft-Start
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
R4
3630 F09
Figure 9. Adjustable UV Lockout
The current that flows through the R3-R4 divider will
directly add to the shutdown, sleep, and active current
of the LTC3630, 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.
The minimum soft-start time is limited to the internal soft-
start timer of 0.8ms. When the LTC3630 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.
Note that the soft-start capacitor may not be the limiting
factor in the output voltage ramp. The maximum output
current, which is equal to half the peak current, must
charge the output capacitor from 0V to its regulated value.
For small peak currents or large output capacitors, this
ramp time can be significant. Therefore, the output voltage
ramp time from 0V to the regulated V_OUT value is limited
to a minimum of:
Ramp Time
≥ ^{2 • COUT}
I_PEAK
V_OUT
C_ISET Selection
Once the peak current resistor, R_ISET, and inductor are se-
lected to meet the load current and frequency requirements,
an optional capacitor, C_ISET, can be added in parallel with
R_ISET. This will boost efficiency at mid-loads and reduce
the output voltage ripple dependency on load current at the
expense of slightly degraded load step transient response.
The peak inductor current is controlled by the voltage on
the I_SET pin. Current out of the I_SET pin is 5µA while the
LTC3630 is switching and is reduced to 1µA during sleep
mode. The I_SET current will return to 5µA on the first cycle
after sleep mode. Placing a parallel RC from the I_SET pin to
ground filters the I_SET voltage as the LTC3630 enters and
exits sleep mode which in turn will affect the output volt-
age ripple, efficiency and load step transient performance.
3630fd
16
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