LM5010ASD datasheet(9/25 Pages) TI

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RON =

VOUT x (VIN - 1.4V)

VIN x FS x 1.18 x 10

-10 - 1.4 k:

RON =

(tON - 67 ns) x (VIN - 1.4V)

1.18 x 10

-10

- 1.4 k:

tON =

1.18 x 10

-10

x (RON + 1.4k)

(VIN - 1.4V)

+ 67 ns

BST

VCC

LM5010A

VOUT

SGND

ISEN

LM5010A

LM5010A-Q1

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SNVS376E – OCTOBER 2005 – REVISED FEBRUARY 2013

The turn-on sequence is shown in Figure 7. When VCC exceeds the under-voltage lock-out threshold (UVLO) of

5.25V (t1 in Figure 7), the buck switch is enabled, and the SS pin is released to allow the soft-start capacitor (C6)

to charge up. The output voltage VOUT is regulated at a reduced level which increases to the desired value as the

soft-start voltage increases (t2 in Figure 7).

Figure 8. 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

programmed by RON, causing the FB voltage to rise above 2.5V. After the on-time period the buck switch

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

The on-time of the internal buck switch is determined by the RON resistor and the input voltage (VIN), and is

calculated as follows:

(5)

The RON resistor can be determined from the desired on-time by re-arranging Equation 5 to the following:

(6)

To set a specific continuous conduction mode switching frequency (fS), the RON resistor is determined from the

following:

(7)

In high frequency applications the minimum value for tON is limited by the maximum duty cycle required for

regulation and the minimum off-time of the LM5010A (260 ns, ±15%). The fixed off-time limits the maximum duty

cycle achievable with a low voltage at VIN. The minimum allowed on-time to regulate the desired VOUT at the

minimum VIN is determined from the following:

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