LTC3632 LINER [Linear Technology], LTC3632 Datasheet - Page 13

• Manufacturer Part Number: LTC3632
• Description: High Effi ciency, High Voltage 20mA Synchronous Step-Down Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The resistive divider allows the V
of the output voltage as shown in Figure 6. Output voltage
adjustment range is from 0.8V to V
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 leak-
age 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:
Run Pin with Programmable Hysteresis
The LTC3632 has a low power shutdown mode controlled
by the RUN pin. Pulling the RUN pin below 0.7V puts the
LTC3632 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.
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
LTC3632
LTC3632
•
GND
V
V
V
OUT
FB
IN
V
FB
OUT
R1
R2
3632 F06
pin to sense a fraction
V
IN
IN
4.7M
.
RUN
LTC3632
3632 F07
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 LTC3632, 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
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.
Rising V
Falling V
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-
V
IN
R1
R2
R3
IN
to ground. The RUN pin com-
RUN
HYST
LTC3632
3632 F08
LTC3632
IN
supply with a
R2
R1
R2 +R3
IN
R1
13
voltage
3632fb