FS =

VOUT

2 x L1 x 1.4 x 1020

RL x RON

2

DC =

tON

tON + tOFF

VOUT

VIN

= tON x FS =

FS =

VOUT x (VIN ± 1.4V)

1.18 x 10

-10

x (RON + 1.4 k:) x VIN

10

LM5010A, LM5010A-Q1

SNVS376F – OCTOBER 2005 – REVISED MAY 2016

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Copyright © 2005–2016, Texas Instruments Incorporated

Feature Description (continued)

The LM5010Ax 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. Calculate the operating frequency in the continuous conduction mode with Equation 2.

(2)

The buck switch duty cycle is equal to Equation 3.

(3)

Under light load conditions, the LM5010Ax 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 because the switching losses diminish as the power delivered

to the load is reduced. Calculate the discontinuous mode operating frequency with Equation 4.

where

•

RL is the load resistance

(4)

7.3.2 Start-Up Regulator (VCC)

A high voltage bias regulator is integrated within the LM5010Ax. The input pin (VIN) can be connected directly to

line voltages between 6 and 75 V. Referring to the block diagram and the graph of VCC vs VIN, when VIN is

between 6 V and the bypass threshold (nominally 8.9 V), the bypass switch (Q2) is on, and VCC tracks VIN within

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

Ω, with inherent current limiting at

approximately 100 mA. When VIN is above the bypass threshold, Q2 is turned off, and VCC is regulated at 7 V.

The VCC regulator output current is limited at approximately 15 mA. When the LM5010Ax is shutdown using the

RON/SD pin, the VCC bypass switch is shut off, regardless of the voltage at VIN.

When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 to 3 µs. The

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 LM5010Ax pins.

In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An

auxiliary voltage of between 7.5 V and 14 V 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 than VIN.

The turn-on sequence is shown in Figure 1. When VCC exceeds the undervoltage lockout threshold (UVLO) of

5.25 V (t1 in Figure 1), 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 1).