LTC3637_15 Datasheet, PDF (15/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
APPLICATIONS INFORMATION
V_OUT
0.8V
V_FB
LTC3637
V_PRG1
V_PRG2
R1
R2
3637 F06
Figure 7. Setting the Output Voltage with External Resistors
To minimize the no-load supply current, resistor values in
the megohm range may be used; however, large resistor
values should be used with caution. The feedback divider
is the only load current when in shutdown. If PCB leakage
current to the output node or switch node exceeds the load
current, the output voltage will be pulled up. In normal
operation, this is generally a minor concern since the load
current is much greater than the leakage.
To avoid excessively large values of R1 in high output volt-
age applications (V_OUT ≥ 10V), a combination of external
and internal resistors can be used to set the output volt-
age. This has an additional benefit of increasing the noise
immunity on the V_FB pin. Figure 8 shows the LTC3637
with the V_FB pin configured for a 5V fixed output with an
external divider to generate a higher output voltage. The
internal 5M resistance appears in parallel with R2, and the
value of R2 must be adjusted accordingly. R2 should be
chosen to be less than 200k to keep the output voltage
variation less than 1% due to the tolerance of the LTC3637’s
internal resistor.
V_OUT
R1
LTC3637 V_{FB 5V}
0.8V
4.2M
800k
SS
V_PRG1
V_PRG2
R2
3637 F08
Figure 8. Setting the Output Voltage with
External and Internal Resistors
RUN Pin and External Input Overvoltage/Undervoltage
Lockout
The RUN pin has two different threshold voltage levels.
Pulling the RUN pin below 0.7V puts the LTC3637 into a
low quiescent current shutdown mode (I_Q ~ 3µA). When
the RUN pin is greater than 1.21V, the controller is enabled.
Figure 9 shows examples of configurations for driving the
RUN pin from logic.
The RUN and OVLO pins can alternatively be configured
as precise undervoltage (UVLO) and overvoltage (OVLO)
lockouts 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 10 to meet specific V_IN voltage requirements.
The current that flows through the R3-R4-R5 divider will
directly add to the shutdown, sleep, and active current of
the LTC3637, and care should be taken to minimize the
impact of this current on the overall efficiency of the ap-
plication circuit. Resistor values in the megohm range may
be required to keep the impact on quiescent shutdown and
sleep currents low. To pick resistor values, the sum total
of R3 + R4 + R5 (R_TOTAL) should be chosen first based on
the allowable DC current that can be drawn from V_IN. The
SUPPLY
LTC3637
RUN
V_IN
LTC3637
RUN
3637 F09
Figure 9. RUN Pin Interface to Logic
V_IN
R3
RUN
R4 ^LTC3637
OVLO
R5
3637 F10
Figure 10. Adjustable UV and OV Lockout
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3637fa
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