LM5010AMH datasheet(9/17 Pages) NSC

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Control Circuit Overview (Continued)

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 resis-

tance 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 operat-

ing frequency in the continuous conduction mode is calcu-

lated as follows:

(2)

The buck switch duty cycle is equal to:

(3)

Under light load conditions, the LM5010A operates in dis-

continuous conduction mode, with zero current flowing

through the inductor for a portion of the off-time. The oper-

ating 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 rela-

tively 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)

where R

L = the load resistance.

Start-Up Bias Regulator (V

CC)

A high voltage bias regulator is integrated within the

LM5010A. The input pin (VIN) can be connected directly to

line voltages between 6V and 75V. Referring to the block

diagram and the graph of V

CC vs. VIN, when VIN is between

6V and the bypass threshold (nominally 8.9V), the bypass

switch (Q2) is on, and V

CC 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 V

CC is regulated at 7V. The VCC regulator output current

is limited at approximately 15 mA. When the LM5010A is

shutdown using the RON/SD pin, the V

CC bypass switch is

shut off, regardless of the voltage at VIN.

When V

IN exceeds the bypass threshold, the time required

for Q2 to shut off is approximately2-3µs. 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 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 2) to shut off the VCC regulator,

reducing internal power dissipation. The current required into

the VCC pin is shown in the Typical Performance Character-

istics. Internally a diode connects VCC to VIN requiring that

the auxiliary voltage be less than V

IN.

The turn-on sequence is shown in Figure 1. When VCC

exceeds the under-voltage lock-out threshold (UVLO) of

5.25V (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 V

OUT is regulated at a reduced level

which increases to the desired value as the soft-start voltage

increases (t2 in Figure 1).

20153816

FIGURE 2. Self Biased Configuration

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