be use to maximize efficiency. When operating the LM2655

in asynchronous mode, the LDR pin should be terminated

with a large resistor (1 Meg

Ω), or left floating. Operation in

asynchronous mode is similar to that of synchronous mode,

except the internal low-side MOSFET logic is not used. At

the beginning of a switching cycle, the high-side MOSFET is

on and current from the input source flows through the induc-

tor and to the load. The current from the high-side MOSFET

is sensed and compared with the output of the error amplifier

(COMP pin). When the sensed current reaches the COMP

pin voltage level, the high-side switch is turned off. At this in-

stant, the load current is commutated through the catch di-

ode. The current now flows through the diode and the induc-

tor and on to the load. At the end of the switching cycle, the

high-side switch is turned on and the cycle is repeated.

Protections

The

peak

current

in

the

system

is

monitored

by

cycle-by-cycle current limit circuitry. This circuitry will turn the

high-side MOSFET off whenever the current through the

high-side MOSFET reaches a preset limit (see plots). A sec-

ond level current limit is accomplished by the undervoltage

protection: if the load pulls the output voltage down below

80% of its nominal value, the undervoltage latch protection

will wait for a period of time (set by the capacitor at the LDE-

LAY pin, see LDELAY CAPACITOR section for more infor-

mation). If the output voltage is still below 80% of its nominal

after the waiting period, the latch protection will be enabled.

In the latch protection mode, the low-side MOSFET is on and

the high-side MOSFET is off. The latch protection will also be

enabled immediately whenever the output voltage exceeds

the overvoltage threshold (110% of its nominal). Both protec-

tions are disabled during start-up.(See SOFT-START CA-

PACITOR section and LDELAY CAPACITOR section for

more information.) Toggling the input supply voltage or the

shutdown pin can reset the device from the latched protec-

tion mode.

DESIGN PROCEDURE

This section presents guidelines for selecting external com-

ponents.

INPUT CAPACITOR

A low ESR aluminum, tantalum, ceramic, or any other type of

capacitor is needed between the input pin and power

ground. This capacitor prevents large voltage transients from

appearing at the input. The capacitor is selected based on

the RMS current and voltage requirements. The RMS cur-

rent is given by:

The RMS current reaches its maximum (I

when

V

the voltage rating should be at least 25% higher than the

maximum input voltage. If a tantalum capacitor is used, the

voltage rating required is about twice the maximum input

voltage. The tantalum capacitor should be surge current

tested by the manufacturer to prevent damage by the inrush

current. It is also recommended to put a small ceramic ca-

pacitor (0.1 µF) between the input pin and ground pin to re-

duce high frequency noise.

INDUCTOR

The most critical parameters for the inductor are the induc-

tance, peak current and the DC resistance. The inductance

is related to the peak-to-peak inductor ripple current, the in-

put and the output voltages:

A higher value of ripple current reduces inductance, but in-

creases the conductance loss, core loss, current stress for

the inductor and switch devices. It also requires a bigger out-

put capacitor for the same output voltage ripple requirement.

A reasonable value is setting the ripple current to be 30% of

the DC output current. Since the ripple current increases

with the input voltage, the maximum input voltage is always

used to determine the inductance. The DC resistance of the

inductor is a key parameter for the efficiency. Lower DC re-

sistance is available with a bigger winding area. A good

tradeoff between the efficiency and the core size is letting the

inductor copper loss equal 2% of the output power.

OUTPUT CAPACITOR

The selection of C

mum allowable output voltage ripple. The output ripple in the

constant frequency, PWM mode is approximated by:

The ESR term usually plays the dominant role in determining

the voltage ripple. A low ESR aluminum electrolytic or tanta-

lum capacitor (such as Nichicon PL series, Sanyo OS-CON,

Sprague 593D, 594D, AVX TPS, and CDE polymer alumi-

num) is recommended. An electrolytic capacitor is not rec-

ommended for temperatures below −25˚C since its ESR

rises dramatically at cold temperature. A tantalum capacitor

has a much better ESR specification at cold temperature and

is preferred for low temperature applications.

The output voltage ripple in constant frequency mode has to

be less than the sleep mode voltage hysteresis to avoid en-

tering the sleep mode at full load:

V

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