2

DC =

1.18 x 10

-10

LM5010A

LM5010A-Q1

SNVS376E – OCTOBER 2005 – REVISED FEBRUARY 2013

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Control Circuit Overview

The LM5010A 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 (2.5V). If the FB voltage is below the reference the

Following the on-time the switch remains off for a fixed 260 ns off-time, or until the FB voltage falls below the

reference, whichever is longer. The buck switch then turns on for another on-time period. Referring to the Block

Diagram, the output voltage is set by R1 and R2. The regulated output voltage is calculated as follows:

(1)

The LM5010A requires a minimum of 25 mV of ripple voltage at the FB pin for stable fixed-frequency operation. If

the output capacitor’s ESR is insufficient additional series resistance may be required (R3 in the Block Diagram).

The LM5010A 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

decaying to 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. The operating frequency in the continuous conduction mode is calculated as follows:

(2)

The buck switch duty cycle is equal to:

(3)

Under light load conditions, the LM5010A operates in discontinuous conduction mode, with zero current flowing

through the inductor for a portion of the off-time. The operating frequency is always lower than that of the

continuous conduction mode, and the switching frequency varies with load current. Conversion efficiency is

maintained at a relatively high level at light loads since the switching losses diminish as the power delivered to

the load is reduced. The discontinuous mode operating frequency is approximately:

(4)

A high voltage bias regulator is integrated within the LM5010A. The input pin (VIN) can be connected directly to

100 mV to 150 mV. The bypass switch on-resistance is approximately 50

Ω, with inherent current limiting at

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 LM5010A 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 8) 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

8

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