LTC3630 Datasheet, PDF (15/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous

LTC3630
APPLICATIONS INFORMATION
The resistive divider allows the V_FB pin to sense a fraction
of the output voltage as shown in Figure 6. The output
voltage can range from 0.8V to V_IN. Be careful to keep the
divider resistors very close to the V_FB pin to minimize the
trace length and noise pick-up on the sensitive V_FB signal.
V_OUT
0.8V
V_FB
LTC3630
V_PRG1
V_PRG2
R1
R2
3630 F06
Figure 6. 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 7 shows the LTC3630
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 LTC3630’s
internal resistor.
V_OUT
R1
LTC3630 V_{FB 5V}
0.8V
4.2M
800k
SS
V_PRG1
V_PRG2
R2
3630 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 LTC3630 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.
SUPPLY
LTC3630
RUN
LTC3630
RUN
3630 F08
Figure 8. RUN Pin Interface to 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.
5V
V_IN
R3
2M
RUN
R4
LTC3630
SLEEP, ACTIVE: 2μA
SHUTDOWN: 0μA
3630 F09
Figure 9. Adjustable UV Lockout
3630fb
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