LTC3642 datasheet(13/20 Pages) LINER

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LTC3642

3642f

APPLICATIONS INFORMATION

The resistive divider allows the VFB pin to sense a fraction

of the output voltage as shown in Figure 6.

VFB

LTC3642

GND

VOUT

Figure 6. Setting the Output Voltage

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:

ΔI

VIN

OUT

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

•

Run Pin with Programmable Hysteresis

The LTC3642 has a low power shutdown mode controlled

by the RUN pin. Pulling the RUN pin below 0.7V puts the

LTC3642 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.

LTC3642

RUN

4.7M

VIN

3642 F07

LTC3642

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.

Speciﬁc values for these UVLO thresholds can be computed

from the following equations:

g V UVLO Threshold

sin

•

⎛

⎝⎜

⎞

⎠

121

2⎟⎟

Fall g V UVLO Threshold

•

110

⎛

⎝⎜

⎞

⎠⎟

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 LTC3642, 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:

IN MAX

() •

⎛

⎝⎜

⎞

⎠⎟

The RUN pin may also be directly tied to the VIN supply

for applications that do not require the programmable

undervoltage lockout feature. In this conﬁguration, switch-

ing is enabled when VINsurpassestheinternalundervoltage

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 ISET pin (25mA to 115mA).

RUN

LTC3642

HYST

VIN

3642 F08

Figure 8. Adjustable Undervoltage Lockout

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