LTC3630_15 Datasheet, PDF (15/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous Step-Down Converter

LTC3630
Applications Information
Output Voltage Programming
The LTC3630 has three fixed output voltage modes that
can be selected with the V_PRG1 and V_PRG2 pins and an
adjustable mode. The fixed output modes use an internal
feedback divider which enables higher efficiency, higher
noise immunity, and lower output voltage ripple for 5V,
3.3V and 1.8V applications. To select the fixed 5V output
voltage, connect V_PRG1 to SS and V_PRG2 to GND. For 3.3V,
connect V_PRG1 to GND and V_PRG2 to SS. For 1.8V, connect
both V_PRG1 and V_PRG2 to SS. For any of the fixed output
voltage options, directly connect the V_FB pin to V_OUT.
For the adjustable output mode (V_PRG1 = 0V, V_PRG2 = 0V),
the output voltage is set by an external resistive divider
according to the following equation:
V_OUT
=
0.8V
•
₁₊
R1
R2 
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
For more information www.linear.com/LTC3630
3630fd
15