LTC3637_15 Datasheet, PDF (16/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
APPLICATIONS INFORMATION
individual values of R3, R4 and R5 can then be calculated
from the following equations:
R5
=
R_TOTAL
•
Rising
V_IN
1.21V
OVLO
Threshold
R4
=
R_TOTAL
•
Rising
V_IN
1.21V
UVLO
Threshold
–
R5
R3 = R_TOTAL –R5 –R4
by the LTC3637 will be optimized by using a catch diode
with minimum reverse leakage current. Low leakage
Schottky diodes often have larger forward voltage drops
at a given current, so a trade-off can exist between low
load and high load efficiency. Often Schottky diodes with
larger reverse bias ratings will have less leakage at a given
output voltage than a diode with a smaller reverse bias
rating. Therefore, superior leakage performance can be
achieved at the expense of diode size.
For applications that do not need a precise external OVLO,
the OVLO pin can be tied directly to ground. The RUN pin
in this type of application can be used as an external UVLO
using the above equations with R5 = 0Ω.
Similarly, for applications that do not require a precise
UVLO, the RUN pin can be tied to V_IN. In this configuration,
the UVLO threshold is limited to the internal V_IN UVLO
thresholds as shown in the Electrical Characteristics table.
The resistor values for the OVLO can be computed using
the above equations with R3 = 0Ω.
Be aware that the OVLO pin cannot be allowed to exceed
its absolute maximum rating of 6V. To keep the voltage
on the OVLO pin from exceeding 6V, the following relation
should be satisfied:
V_{IN(MAX )}
•


R5 _
R3 + R4 + R5 ^
<
6V
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
The minimum soft-start time is limited to the internal soft-
start timer of 0.8ms. When the LTC3637 detects a fault
condition (input supply undervoltage or overtemperature)
or when the RUN pin falls below 1.1V, or when the OVLO
pin rises above 1.21V, 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.
Catch Diode Selection
The catch diode D1 conducts current only during switch-off
time. Use a Schottky diode to limit forward voltage drop to
increase efficiency. The Schottky diode must have a peak
reverse voltage that is equal to the regulator maximum
input voltage or OVLO set voltage and must be sized for
average forward current in normal operation. Average
forward current can be calculated from:
( ) ^{ID(AVG) =}
I_OUT • V_IN
V_IN – V_OUT
An additional consideration is reverse leakage current.
When the catch diode is reversed biased, any leakage
current will appear as load current. When operating under
light load conditions, the low supply current consumed
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
Optimizing Output Voltage Ripple
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
3637fa
16
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