LTC4366 datasheet(20/32 Pages) AD

20 / 32 page

LTC7862

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 5. Adjustable UV and OV Lockout

The RUN and OVLO pins can alternatively be configured

as undervoltage (UVLO) and overvoltage (OVLO) lock-

outs on the VIN supply with a resistor divider from VIN to

ground. A simple resistor divider can be used as shown

in Figure 5 to meet specific VIN voltage requirements.

RUN

7862 F05

VIN

LTC7862

OVLO

For applications that do not require a precise OVLO, the

OVLO pin can be tied directly to ground. The RUN pin in

this type of application can be used as an external UVLO

using the previous equations with R5 = 0Ω.

Similarly, for applications that do not require a precise

UVLO, the RUN pin can be tied to VIN. In this configura-

tion, the UVLO threshold is limited to the internal DRVCC

UVLO thresholds as shown in the Electrical Characteristics

table. The resistor values for the OVLO can be computed

using the previous equations with R3 = 0Ω.

Soft-Start (SS) Pin

The start-up of VOUT is controlled by the voltage on the

SS pin. When the voltage on the SS pin is less than the

internal 0.8V reference, the LTC7862 regulates the VFB pin

voltage to the voltage on the SS pin instead of the internal

reference. The SS pin can be used to program an external

soft-start function.

Soft-start is enabled by simply connecting a capacitor

from the SS pin to ground, as shown in Figure 6. An inter-

nal 10μA current source charges the capacitor, providing

a linear ramping voltage at the SS pin. The LTC7862 will

regulate its feedback voltage (and hence VOUT) accord-

ing to the voltage on the SS pin, allowing VOUT to rise

smoothly from 0V to its final regulated value. The total

soft-start time will be approximately:

tSS = CSS •

0.8V

10µA

The current that flows through the R3-R4-R5 divider will

directly add to the shutdown, sleep, and active current

of the LTC7862, and care should be taken to minimize

the impact of this current on the overall efficiency of the

application circuit. Resistor values in the megaohm range

may be required to keep the impact on quiescent shut-

down and sleep currents low. To pick resistor values, the

sum total of R3 + R3+ R5 (RTOTAL) should be chosen

first based on the allowable DC current that can be drawn

from VIN.

The individual values of R3, R4 and R5 can be calculated

from the following equations:

= RTOTAL •

1.20V

RISING VIN OVLO THRESHOLD

= RTOTAL •

1.20V

RISING VIN RUN THRESHOLD

−R5

= RTOTAL −R5−R4

Figure 6. Using the SS Pin to Program Soft-Start

7862 F06

LTC7862

GND

CSS

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