LM46001 datasheet(32/56 Pages) TI2

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LM46001

SNVSA46B – JUNE 2014 – REVISED JANUARY 2018

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8.2.2.11 Soft-Start Capacitors

The user can leave the SS/TRK pin floating, and the LM46001 implements a soft-start time of 4.1 ms typically. In

order to use an external soft start capacitor, choose the capacitor size so that the soft-start time is longer than

4.1 ms. Use Equation 23 to calculate the soft-start capacitor value:

where

•

CSS = Soft-start capacitor value (µF)

•

ISS = Soft-start charging current (µA)

•

tSS = Desired soft start time (s)

(23)

For the desired soft-start time of 10 ms and soft-start charging current of 2.2 µA, Equation 23 yields a soft-start

capacitor value of 0.022 µF.

8.2.2.12 Undervoltage Lockout Setpoint

The undervoltage lockout (UVLO) is adjusted using the external voltage divider network of RENT and RENB. RENT

is connected between VIN and the EN pin of the LM46001 device. RENB is connected between the EN pin and

the GND pin. The UVLO has two thresholds, one for power up when the input voltage is rising and one for power

down or brownouts when the input voltage is falling. Use Equation 24 to determine the rising VIN (UVLO) level:

VIN-UVLO-RISING = VENH × (RENB + RENT) / RENB

(24)

The EN rising threshold for LM46001 is set to be 2.1 V. Choose the value of RENB to be 1 MΩ to minimize input

current going into the converter. If the desired VIN (UVLO) level is at 5 V, then the value of RENT can be

calculated using Equation 25:

RENT = (VIN-UVLO-RISING / VENH – 1) × RENB

(25)

Equation 25 yields a value of 1.37 M

Ω. The resulting falling UVLO threshold can be calculated by Equation 26:

VIN-UVLO-FALLING = 1.8 × (RENB + RENT) / RENB

(26)

8.2.2.13 PGOOD

A typical pullup resistor value is 10 k

Ω to 100 kΩ from the PGOOD pin to a voltage no higher than 12 V. If it is

desired to pull up the PGOOD pin to a voltage higher than 12 V, a resistor can be added from the PGOOD pin to

ground to divide the voltage seen by the PGOOD pin to a value no higher than 12 V.

LM46001 Datasheet(PDF) 32 Page - Texas Instruments