Datasheet

Operation
The LM2655 is a constant frequency (300kHz), current-
mode PWM switcher that can be operated synchronously or
asynchronously.
SYNCHRONOUS OPERATION
A converter is said to be in synchronous operation when a
MOSFET is used in place of the catch diode. In the case of
the buck converter, this MOSFET is known as the low-side
MOSFET (the MOSFET connected between the input
source and the low-side MOSFET is the high-side MOS-
FET). Converters in synchronous operation exhibit higher
efficiencies compared to asynchronous operation because
the I
2
R losses are reduced with the use of a MOSFET .
Operation of the LM2655 in synchronous mode is identical to
its operation in asynchronous mode, except that internal
logic drives the low-side MOSFET. At the beginning of a
switching cycle, the high-side MOSFET is on and current
from the input source flows through the inductor 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. After a 30ns delay
(deadtime), the low-side driver goes high and turns the
low-side MOSFET on. The current now flows through the
low-side MOSFET, through the inductor and on to the load. A
30ns delay is necessary to insure that the MOSFETs are
never on at the same time. During the 30ns deadtime, the
current is forced to flow through the low-side MOSFET’s
body diode. It is recommended that a low forward drop
schottky diode be placed in parallel to the low-side MOSFET
so that current will be more efficiently conducted during this
30ns deadtime. This Schottky diode should be placed within
5mm of the switch pin so that current limit is not effected (see
External Schottky Diode section). At the end of the switching
cycle, the low-side switch is turned off and after another
30ns delay, the cycle is repeated.
Current through the high-side MOSFET is sensed by pat-
ented circuitry that does not require an external sense resis-
tor. As a result, system cost and size are reduced, efficiency
is increased, and noise immunity of the sensed current is
improved. A feedforward from the input voltage is added to
reduce the variation of the current limit over the input voltage
range.
ASYNCHRONOUS OPERATION
A unique feature of the LM2655 is that it can be operated in
either synchronous or asynchronous mode. When operating
in asynchronous mode, a small amount of efficiency is sac-
rificed for a less expensive solution. Any diode may be used,
but it is recommended that a low forward drop schottky diode
be use to maximize efficiency. When operating the LM2655
in asynchronous mode, the LDR pin should be terminated
with a large resistor (
>
1 MegΩ), 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
inductor 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 instant, the load current is commutated
through the catch diode. The current now flows through the
diode and the inductor 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 second
level current limit is accomplished by the undervoltage pro-
tection: 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 LDELAY
pin, see LDELAY CAPACITOR section for more information).
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 pro-
tections 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
OUT
/2) when
V
IN
equals 2V
OUT
. For an aluminum or ceramic capacitor,
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
capacitor (0.1 µF) between the input pin and ground pin to
reduce 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
input and the output voltages:
A higher value of ripple current reduces inductance, but
increases the conductance loss, core loss, current stress for
the inductor and switch devices. It also requires a bigger
output capacitor for the same output voltage ripple require-
LM2655
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