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

FB

SW

L1

C2

R1

R2

R3

BST

VCC

D2

C3

C4

D1

LM5010A

VOUT

SGND

ISEN

11

LM5010A, LM5010A-Q1

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SNVS376F – OCTOBER 2005 – REVISED MAY 2016

Product Folder Links: LM5010A LM5010A-Q1

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Copyright © 2005–2016, Texas Instruments Incorporated

Feature Description (continued)

Figure 8. Self Biased Configuration

7.3.3 Regulation Comparator

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

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

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

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

temperature.

7.3.4 Overvoltage Comparator

The feedback voltage at FB is compared to an internal 2.9-V reference. If the voltage at FB rises above 2.9 V 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 2.5 V.

7.3.5 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 with Equation 5.

(5)

The RON resistor can be determined from the desired ON-time by re-arranging Equation 5 to Equation 6.

(6)

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

Equation 7.

(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 LM5010Ax (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 with Equation 8.