LTC3631 Datasheet, PDF (13/22 Pages) Linear Technology – High Effi ciency, High Voltage 100mA Synchronous Step-Down Converter

LTC3631
APPLICATIONS INFORMATION
The resistive divider allows the V_FB pin to sense a fraction
of the output voltage as shown in Figure 6. Output voltage
can range from 0.8V to V_IN.
V_OUT
V_FB
LTC3631
GND
R1
R2
3631 F06
Figure 6. Setting the Output Voltage
To minimize the no-load supply current, resistor values
in the megohm range should 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. The
increase in supply current due to the feedback resistors
can be calculated from:

I_VIN
=
 _{RV1O+URT 2}



•



V_OUT
V_IN



Run Pin with Programmable Hysteresis
The LTC3631 has a low power shutdown mode controlled
by the RUN pin. Pulling the RUN pin below 0.7V puts the
LTC3631 into a low quiescent current shutdown mode
(I_Q ~ 3μA). When the RUN pin is greater than 1.2V,
the controller is enabled. Figure 7 shows examples of
configurations for driving the RUN pin from logic.
V_SUPPLY
V_IN
LTC3631
RUN
4.7M LTC3631
RUN
3631 F07
Figure 7. RUN Pin Interface to Logic
The RUN pin can alternatively be configured as a precise
undervoltage lockout (UVLO) on the V_IN supply with
a resistive divider from V_IN to ground. The RUN pin
comparator nominally provides 10% hysteresis when
used in this method; however, additional hysteresis may
be added with the use of the HYST pin. The HYST pin is
an open-drain output that is pulled to ground whenever
the RUN comparator is not tripped. A simple resistive
divider can be used as shown in Figure 8 to meet specific
V_IN voltage requirements.
V_IN
R1
RUN
R2
LTC3631
HYST
R3
3631 F08
Figure 8. Adjustable Undervoltage Lockout
Specific values for these UVLO thresholds can be computed
from the following equations:
Rising
V_IN
UVLO
Threshold = 1.21V


• _1+

R1^
R2


Falling
V_IN
UVLO
Threshold = 1.10V
•


¹⁺

R1 ^
R2
+
R3


The minimum value of these thresholds is limited to the
internal V_IN UVLO thresholds that are shown in the Electrical
Characteristics table. The current that flows through this
divider will directly add to the shutdown, sleep and active
current of the LTC3631, and care should be taken to
minimize the impact of this current on the overall efficiency
of the application circuit. Resistor values in the megohm
range may be required to keep the impact on quiescent
shutdown and sleep currents low. Be aware that the HYST
pin cannot be allowed to exceed its absolute maximum
rating of 6V. To keep the voltage on the HYST pin from
exceeding 6V, the following relation should be satisfied:
V_IN(MAX)
•




R1+
R3
R2 +
R3



<
6V
3631fb
13