LTM8048 datasheet(14/22 Pages) LINER

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LTM8045

8045fa

For more information www.linear.com/8045

APPLICATIONS INFORMATION

Soft-Start

The LTM8045 soft-start function controls the slew rate

of the power supply output voltage during start-up. A

controlled output voltage ramp minimizes output voltage

overshoot, reduces inrush current from the VIN supply,

and facilitates supply sequencing. A capacitor connected

from the SS pin to GND programs the slew rate. In the

event of a commanded shutdown or lockout (RUN pin),

internal undervoltage lockout or a thermal shutdown, the

soft-start capacitor is automatically discharged before

charging resumes, thus assuring that the soft-start occurs

when the LTM8045 restarts. The soft-start time is given

by the equation:

tSS = CSS/5.45,

where CSS is in µF and tSS is in seconds.

Configurable Undervoltage Lockout

Figure 1 shows how to configure an undervoltage lock-

out (UVLO) for the LTM8045. Typically, UVLO is used in

situations where the input supply is current-limited, has

a relatively high source resistance, or ramps up/down

slowly. A switching regulator draws constant power from

the source, so source current increases as source voltage

drops. This looks like a negative resistance load to the

source and can cause the source to current-limit or latch

low under low source voltage conditions. UVLO prevents

the regulator from operating at source voltages where

these problems might occur.

The RUN pin has a voltage hysteresis with typical thresh-

olds of 1.32V (rising) and 1.29V (falling) and an internal

circuit that draws typically 11.6µA at the RUN threshold.

This makes RUVLO2 optional, allowing UVLO implemen-

tation with a single resistor. Resistor RUVLO2 is optional.

RUVLO2canbeincludedtoreducetheoverallUVLOvoltage

variation caused by variations in the RUN pin current (see

the Electrical Characteristics section). A good choice for

RUVLO2 is ≤10k ±1%. After choosing a value for RUVLO2,

RUVLO1 can be determined from either of the following:

RUVLO1 =

VIN(RISING) − 1.32V

1.32V

RUVLO2

+ 11.6µA

RUVLO1 =

VIN(FALLING) − 1.29V

1.29V

RUVLO2

+ 11.6µA

where VIN(RISING) and VIN(FALLING) are the VIN threshold

voltages when rising or falling, respectively.

For example, to disable the LTM8045 for VIN voltages

below3.5Vusingthesingleresistorconfiguration,choose:

RUVLO1 =

3.5V

− 1.29V

1.29V

∞

+ 11.6µA

= 191k

To activate the LTM8045 for VIN voltage greater than 4.5V

using the two resistor configuration, choose RUVLO2 =

10k and:

RUVLO1 =

4.5V

− 1.32V

1.32V

10k

+ 11.6µA

= 22.1k

Internal Undervoltage Lockout

The LTM8045 monitors the VIN supply voltage in case VIN

drops below a minimum operating level (typically about

2.3V). When VIN is detected low, the power switch is

deactivated, and while sufficient VIN voltage persists, the

soft-startcapacitorisdischarged.AfterVINisdetectedhigh,

the LTM8045 will reactivate and the soft-start capacitor

will begin charging.

LTM8045

GND

VIN

RUN

RUVLO1

RUVLO2

VIN

8045 F01

Figure 1. The RUN Pin May Be Used

to Implement an Accurate UVLO

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