LTM8049 datasheet(12/20 Pages) LINER

12 / 20 page

LTM8049

8049fa

For more information www.linear.com/LTM8049

Soft-Start

The soft-start circuitry provides for a gradual ramp-up of

the switch current in each channel. When the channel is

enabled, the external SS capacitor is first discharged. This

resets the state of the logic circuits in the channel. Then

an integrated resistor pulls the channel’s SS pin to about

1.8V. The LTM8049 has a built-in soft-start characteristic,

but a slower ramp rate may be implemented by adding

capacitance to the SS pin. Typical values are between

0.1µF and 1µF.

Configurable Undervoltage Lockout

Figure 1 shows how to configure an undervoltage lock-

out (UVLO) for the LTM8049. 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.

APPLICATIONS INFORMATION

The RUN pin has a voltage hysteresis with typical thresh-

olds of 1.31V (rising) and 1.27V (falling). Resistor RUVLO2

is optional. RUVLO2 can be included to reduce the overall

UVLO voltage variation caused by variations in the RUN

pin current (see the Electrical Characteristics). 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) – 1.31V

1.32V

RUVLO2

+ 12.1µA

RUVLO1 =

VIN(FALLING) – 1.27V

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 LTM8049 for VIN voltages

below3.5Vusingthesingleresistorconfiguration,choose:

RUVLO1 =

3.5V – 1.27V

1.29V

∞

+ 11.6µA

≅ 191k

To activate the LTM8049 for VIN greater than 4.5V using

the two resistor configuration, choose RUVLO2 = 10k and:

RUVLO1 =

4.5V – 1.31V

1.32V

10k

+ 12.1µA

≅ 22.1k

Internal Undervoltage Lockout

The LTM8049 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-start capacitor is discharged. After VIN is sufficiently

high,theLTM8049willreactivateandthesoft-startcapaci-

tor will begin charging.

The RUN pin sinks 12.1µA at the 1.31V rising threshold

voltage and about 11.6µA at the 1.27V falling threshold.

This makes it easy to set up an input voltage UVLO thresh-

old with just a single resistor. For a desired VIN threshold,

choose RUVLO1 using the equation:

RUVLO1 =

VIN – 1.31V

12.1µA

Figure 1. The RUN Pin May Be Used to Implement

an Accurate UVLO

LTM8049

ACTIVE/

LOCKOUT

8049 F01

RUVLO1

RUVLO2

(OPTIONAL)

VIN

RUN

1.31V

12.3µA

AT 1.31V

GND

–

LTM8049 Datasheet(PDF) 12 Page - Linear Technology

LTM8049 Datasheet(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL