LM2694 datasheet(12/19 Pages) NSC

12 / 19 page

Current Limit

Current limit detection occurs during the off-time by monitor-

ing the recirculating current through the free-wheeling diode

(D1). Referring to Figure 7, when the buck switch is turned

off the inductor current flows through the load, into SGND,

through the sense resistor, out of ISEN and through D1. If

that current exceeds 0.62A the current limit comparator out-

put switches to delay the start of the next on-time period if

the voltage at FB is below 2.5V. The next on-time starts

when the current out of ISEN is below 0.62A and the voltage

at FB is below 2.5V. If the overload condition persists caus-

ing the inductor current to exceed 0.62A during each on-

time, that is detected at the beginning of each off-time. The

operating frequency is lower due to longer-than-normal off-

times.

Figure 11 illustrates the inductor current waveform. During

normal operation the load current is Io, the average of the

ripple waveform. When the load resistance decreases the

current ratchets up until the lower peak reaches 0.62A.

During the Current Limited portion of Figure 11, the current

ramps down to 0.62A during each off-time, initiating the next

on-time (assuming the voltage at FB is <2.5V). During each

on-time the current ramps up an amount equal to:

∆I=(V

IN -VOUT)xtON /L1

During this time the LM2694 is in a constant current mode,

with an average load current (I

OCL) equal to 0.62A +

∆I/2.

N - Channel Buck Switch and

Driver

The LM2694 integrates an N-Channel buck switch and as-

sociated floating high voltage gate driver. The peak current

allowed through the buck switch is 1.5A, and the maximum

allowed average current is 1A. The gate driver circuit works

in conjunction with an external bootstrap capacitor and an

internal high voltage diode. A 0.022 µF capacitor (C4) con-

nected between BST and SW provides the voltage to the

driver during the on-time. During each off-time, the SW pin is

at approximately -1V, and C4 charges from V

CC through the

internal diode. The minimum off-time of 265 ns ensures a

minimum time each cycle to recharge the bootstrap capaci-

tor.

Softstart

The softstart feature allows the converter to gradually reach

a steady state operating point, thereby reducing start-up

stresses and current surges. Upon turn-on, after V

reaches the under-voltage threshold, an internal 12 µA cur-

rent source charges up the external capacitor at the SS pin

to 2.5V. The ramping voltage at SS (and the non-inverting

input of the regulation comparator) ramps up the output

voltage in a controlled manner.

An internal switch grounds the SS pin if V

CC is below the

under-voltage lockout threshold, if a thermal shutdown oc-

curs, or if the RON/SD pin is grounded.

Thermal Shutdown

The LM2694 should be operated so the junction temperature

does not exceed 125˚C. If the junction temperature in-

creases, an internal Thermal Shutdown circuit, which acti-

vates (typically) at 175˚C, takes the controller to a low power

reset state by disabling the buck switch and the on-timer,

and grounding the Softstart pin. This feature helps prevent

catastrophic failures from accidental device overheating.

When the junction temperature reduces below 155˚C (typical

hysteresis = 20˚C), the Softstart pin is released and normal

operation resumes.

Applications Information

EXTERNAL COMPONENTS

The procedure for calculating the external components is

illustrated with a design example. Referring to the Block

Diagram, the circuit is to be configured for the following

specifications:

•

OUT =5V

•

IN =8Vto30V

20187020

FIGURE 11. Inductor Current - Current Limit Operation

LM2694 Datasheet(PDF) 12 Page - National Semiconductor (TI)

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