LM2694 datasheet(10/19 Pages) NSC

10 / 19 page

Functional Description

The LM2694 Step Down Switching Regulator features all the

functions needed to implement a low cost, efficient buck bias

power converter capable of supplying at least 0.6A to the

load. This high voltage regulator contains a 30V N-Channel

buck switch, is easy to implement, and is available in the

TSSOP-14 and the thermally enhanced LLP-10 packages.

The regulator’s operation is based on a constant on-time

control scheme, where the on-time is determined by V

IN.

This feature allows the operating frequency to remain rela-

tively constant with load and input voltage variations. The

feedback control requires no loop compensation resulting in

very fast load transient response. The valley current limit

detection circuit, internally set at 0.62A, holds the buck

switch off until the high current level subsides. This scheme

protects against excessively high currents if the output is

short-circuited when V

IN is high. The functional block dia-

gram is shown in Figure 7.

The LM2694 can be applied in numerous applications to

efficiently regulate down higher voltages. Additional features

include: Thermal shutdown, V

CC under-voltage lockout, gate

drive under-voltage lockout, and maximum duty cycle limiter.

Control Circuit Overview

The LM2694 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 inter-

nal reference (2.5V). 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 (R

ON). Follow-

ing the on-time the switch remains off for a minimum of 265

ns, and until the FB voltage falls below the reference. The

buck switch then turns on for another on-time period. Typi-

cally, during start-up, or when the load current increases

suddenly, the off-times are at the minimum of 265 ns. Once

regulation is established, the off-times are longer.

When in regulation, the LM2694 operates in continuous

conduction mode at heavy load currents and discontinuous

conduction mode at light load currents. In continuous con-

duction 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. The operating frequency is approximately:

(1)

The buck switch duty cycle is equal to:

(2)

In discontinuous conduction mode current through the induc-

tor 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 continu-

ous conduction mode, and varies with load current. Conver-

sion efficiency is maintained at light loads since the switch-

ing losses reduce with the reduction in load and frequency.

The approximate discontinuous operating frequency can be

calculated as follows:

(3)

where R

L = the load resistance.

The output voltage is set by two external resistors (R1, R2).

The regulated output voltage is calculated as follows:

OUT =2.5x(R1+R2)/R2

Output voltage regulation is based on ripple voltage at the

feedback input, requiring a minimum amount of ESR for the

output capacitor C2. The LM2694 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 Figure 7).

Start-Up Regulator, V

The start-up regulator is integral to the LM2694. The input

pin (VIN) can be connected directly to line voltage up to 30V,

with transient capability to 33V. The V

CC output regulates at

7.0V, and is current limited at 9 mA. Upon power up, the

regulator sources current into the external capacitor at VCC

(C3). When the voltage on the VCC pin reaches the under-

voltage lockout threshold of 5.7V, the buck switch is enabled

and the Softstart pin is released to allow the Softstart capaci-

tor (C6) to charge up.

The minimum input voltage is determined by the regulator’s

dropout voltage, the V

CC UVLO falling threshold (

)5.5V),

and the frequency. When V

CC falls below the falling thresh-

old the V

CC UVLO activates to shut off the output. If VCC is

externally loaded, the minimum input voltage increases.

To reduce power dissipation in the start-up regulator, an

auxiliary voltage can be diode connected to the V

CC pin.

Setting the auxiliary voltage to between 8V and 14V shuts off

the internal regulator, reducing internal power dissipation.

The sum of the auxiliary voltage and the input voltage (V

IN) cannot exceed 47V. Internally, a diode connects VCC

to VIN. See Figure 9.

LM2694 Datasheet(PDF) 10 Page - National Semiconductor (TI)

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