LTC7801 datasheet(21/38 Pages) LINER

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LTC7801

7801f

For more information www.linear.com/LTC7801

applicaTions inForMaTion

Figure 5. Adjustable UV and OV Lockout

The RUN and OVLO pins can alternatively be configured as

undervoltage(UVLO)andovervoltage(OVLO)lockoutson

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

7801 F05

VIN

LTC7801

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

UVLOthresholdsasshownintheElectricalCharacteristics

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 LTC7801 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-startisenabledbysimplyconnectingacapacitorfrom

the SS pin to ground, as shown in Figure 6. An internal

10μA current source charges the capacitor, providing a

linear ramping voltage at the SS pin. The LTC7801 will

regulate its feedback voltage (and hence VOUT) according

tothevoltageontheSSpin,allowingVOUTtorisesmoothly

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 LTC7801, and care should be taken to minimize the

impact of this current on the overall efficiency of the ap-

plication circuit. Resistor values in the megaohm range

mayberequiredtokeeptheimpactonquiescentshutdown

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

total of R3 + R3+ R5 (RTOTAL)shouldbechosenfirstbased

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 OVLO THRESHOLD

−R5

= RTOTAL −R5−R4

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

7801 F06

LTC7801

GND

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