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COUT

RFB2

VOUT

LM5017

RFB1

VOUT

(low ripple)

RFB2 + RFB1

VOUT = 1.225V x

RFB1

VOUT

CSW =

K x RON

LM5017

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SNVS783I – JANUARY 2012 – REVISED OCTOBER 2015

7.3 Feature Description

7.3.1 Control Overview

The LM5017 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the

output voltage feedback (FB) compared to an internal reference (1.225 V). If the FB voltage is below the

reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input

voltage and the programming resistor (RON). Following the on-time the switch remains off until the FB voltage

falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer.

When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck

switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB

voltage is approximately equal to 1.225 V (typ).

In a synchronous buck converter, the low side (sync) FET is ‘on’ when the high side (buck) FET is ‘off’. The

inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.

There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative

direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of

the output loading. The operating frequency remains relatively constant with load and line variations. The

operating frequency can be calculated as shown in Equation 1.

(1)

Where:

K = 9 x 10–11

The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated

as shown in Equation 2.

(2)

This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum

amount of ESR for the output capacitor (COUT). A minimum of 25 mV of ripple voltage at the feedback pin (FB) is

required for the LM5017. In cases where the capacitor ESR is too small, additional series resistance may be

required (RC in Figure 10).

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 10. However, RC slightly degrades the load regulation.

Figure 10. Low Ripple Output Configuration

7.3.2 VCC Regulator

The LM5017 device contains an internal high voltage linear regulator with a nominal output of 7.6 V. The input

pin (VIN) can be connected directly to the line voltages up to 100 V. The VCC regulator is internally current limited

to 30 mA. The regulator sources current into the external capacitor at VCC. This regulator supplies current to

internal circuit blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the

VCC pin reaches the undervoltage lockout (VCC UVLO) threshold of 4.5 V, the IC is enabled.

An internal diode connected from VCC to the BST pin replenishes the charge in the gate drive bootstrap capacitor

when SW pin is low.

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