LTC3631 Linear Technology Corporation, LTC3631 Datasheet - Page 13

• Manufacturer Part Number: LTC3631
• Description: High Voltage 100mA Synchronous Step-Down Converter
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
The resistive divider allows the V
of the output voltage as shown in Figure 6.
To minimize the no-load supply current, resistor values in
the megohm range should be used. The increase in supply
current due to the feedback resistors can be calculated
from:
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
controller is enabled. Figure 7 shows examples of con-
fi gurations for driving the RUN pin from logic.
The RUN pin can alternatively be confi gured as a precise
undervoltage lockout (UVLO) on the V
resistive divider from V
parator 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 specifi c V
requirements.
Q
~ 3μA). When the RUN pin is greater than 1.2V, the
I
VIN
V
SUPPLY
=
R1+ R2
Figure 7. RUN Pin Interface to Logic
Figure 6. Setting the Output Voltage
V
OUT
RUN
LTC3631
LTC3631
•
IN
GND
V
V
V
FB
OUT
to ground. The RUN pin com-
IN
V
FB
OUT
R1
R2
3631 F06
pin to sense a fraction
V
IN
4.7M
IN
RUN
supply with a
LTC3631
3631 F07
IN
voltage
Specifi c values for these UVLO thresholds can be computed
from the following equations:
The minimum value of these thresholds is limited to the
internal V
cal Characteristics table. The current that fl ows 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 effi ciency
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 satisfi ed:
The RUN pin may also be directly tied to the V
for applications that do not require the programmable
undervoltage lockout feature. In this confi guration, switch-
ing is enabled when V
lockout threshold.
Soft-Start
The internal 0.75ms soft-start is implemented by ramping
both the effective reference voltage from 0V to 0.8V and the
peak current limit set by the I
Rising V
Falling V
V
IN(MAX)
IN
IN
IN
Figure 8. Adjustable Undervoltage Lockout
UVLO Threshold = 1.21V • 1+
UVLO Threshold = 1.10V • 1+
UVLO thresholds that are shown in the Electri-
•
R1+ R2 + R3
R3
IN
V
IN
surpasses the internal undervoltage
R1
R2
R3
RUN
HYST
LTC3631
< 6V
SET
3631 F08
pin (50mA to 225mA).
LTC3631
R2
R1
R2 + R3
IN
R1
13
supply
3631fa