LTC3631 datasheet(13/22 Pages) LINER

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LTC3631

3631fb

APPLICATIONS INFORMATION

The resistive divider allows the VFB pin to sense a fraction

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

can range from 0.8V to VIN.

VFB

LTC3631

GND

VOUT

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:

IVIN =

VOUT

R1+ R2

•

VOUT

VIN

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

(IQ ~ 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.

LTC3631

RUN

4.7M

VIN

3631 F07

LTC3631

RUN

VSUPPLY

Figure 7. RUN Pin Interface to Logic

RUN

LTC3631

HYST

VIN

3631 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 VINUVLOthresholdsthatareshownintheElectrical

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:

VIN(MAX) •

R1+ R2 + R3

< 6V

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

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

VIN voltage requirements.

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