1.18 x 10

-10

- 1.4 k:

1.18 x 10

-10

+ 67 ns

FB

SW

L1

C2

R1

R2

R3

BST

VCC

D2

C3

C4

D1

LM25010

VOUT

SGND

ISEN

LM25010

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SNVS419C – DECEMBER 2005 – REVISED DECEMBER 2008

capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its

absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to

the LM25010 pins.

In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An

auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin (D2 in Figure 2) to shut off the

VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the Typical

Performance Characteristics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less

The turn-on sequence is shown in Figure 1. When VCC exceeds the under-voltage lock-out threshold (UVLO) of

5.25V (t1 in Figure 1), the buck switch is enabled, and the SS pin is released to allow the soft-start capacitor (C6)

soft-start voltage increases (t2 in Figure 1).

Figure 2. Self Biased Configuration

Regulation Comparator

The feedback voltage at the FB pin is compared to the voltage at the SS pin (2.5V, ±2%). In normal operation an

on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch conducts for the on-time

remains off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 5 nA over

temperature.

Over-Voltage Comparator

The feedback voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the

on-time is immediately terminated. This condition can occur if the input voltage, or the output load, changes

suddenly. The buck switch remains off until the voltage at FB falls below 2.5V.

ON-Time Control

calculated as follows:

(5)

(6)

following:

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