FS =

VOUT

2 x L1 x 1.4 x 1020

RL x (RON)

2

DC =

tON

tON + tOFF

VOUT

VIN

=

FS =

VOUT

1.18 x 10

-10 x R

ON

10

LM5010

SNVS307G – SEPTEMBER 2004 – REVISED APRIL 2016

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Copyright © 2004–2016, Texas Instruments Incorporated

Feature Description (continued)

7.3.1 Control Circuit Overview

The LM5010 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot ON timer,

with the output voltage feedback (FB) compared to an internal reference (2.5 V). If the FB voltage is below the

reference the buck switch is turned on for a time period determined by the input voltage and a programming

resistor (RON). Following the ON-time the switch remains off for 265 ns, or until the FB voltage falls below the

reference, whichever is longer. The buck switch then turns on for another ON-time period. Typically when the

load current increases suddenly, the OFF-times are temporarily at the minimum of 265 ns. Once regulation is

established, the OFF-time resumes its normal value. The output voltage is set by two external resistors (R1, R2).

The regulated output voltage is calculated with Equation 1.

VOUT = 2.5 V × (R1 + R2) / R2

(1)

Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR

for the output capacitor C2. The LM5010 requires a minimum of 25-mV of ripple voltage at the FB pin. In cases

where the capacitor’s ESR is insufficient, additional series resistance may be required (R3 in Functional Block

Diagram).

When in regulation, the LM5010 operates in continuous conduction mode at heavy load currents and

discontinuous conduction mode at light load currents. In continuous conduction mode current always flows

through the inductor, never reaching zero during the OFF-time. In this mode the operating frequency remains

relatively constant with load and line variations. The minimum load current for continuous conduction mode is

one-half the inductor’s ripple current amplitude. Calculate the approximate operating frequency with Equation 2.

(2)

The buck switch duty cycle is approximately equal to Equation 3.

(3)

At low load current, the circuit operates in discontinuous conduction mode, during which the inductor current

ramps up from zero to a peak during the ON-time, then ramps back to zero before the end of the OFF-time. The

next ON-time period starts when the voltage at FB falls below the reference until then the inductor current

remains zero, and the load current is supplied by the output capacitor (C2). In this mode the operating frequency

is lower than in continuous conduction mode, and varies with load current. Conversion efficiency is maintained at

light loads because the switching losses reduce with the reduction in load and frequency. Calculate the

approximate discontinuous operating frequency with Equation 4.

where

•

RL = the load resistance

(4)

For applications where lower output voltage ripple is required, the output can be taken directly from a low ESR

output capacitor as shown in Figure 8. However, R3 slightly degrades the load regulation.