LTC3632 datasheet(13/22 Pages) LINER

13 / 22 page

LTC3632

3632fb

APPLICATIONS INFORMATION

The resistive divider allows the VFB pin to sense a fraction

of the output voltage as shown in Figure 6. Output voltage

adjustment range is from 0.8V to VIN.

VFB

LTC3632

GND

VOUT

3632 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 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:

IVIN =

VOUT

R1+R2

•

VOUT

VIN

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

(IQ ~ 3μA). When the RUN pin is greater than 1.2V, the

controller is enabled. Figure 7 shows examples of con-

ﬁgurations for driving the RUN pin from logic.

LTC3632

RUN

4.7M

VIN

3632 F07

LTC3632

RUN

VSUPPLY

Figure 7. RUN Pin Interface to Logic

The RUN pin can alternatively be conﬁgured as a precise

undervoltage lockout (UVLO) on the VIN supply with a

resistive divider from VIN to ground. The RUN pin com-

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 speciﬁc VIN voltage

requirements.

RUN

LTC3632

HYST

VIN

3632 F08

Figure 8. Adjustable Undervoltage Lockout

Speciﬁc values for these UVLO thresholds can be computed

from the following equations:

Rising VIN UVLO Threshold = 1.21V • 1+

Falling VIN UVLO Threshold = 1.10V • 1+

R2 +R3

The minimum value of these thresholds is limited to the

internal VIN UVLO thresholds that are shown in the Electri-

cal Characteristics table. The current that ﬂows 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 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

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