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LM2655 Datasheet(PDF) 8 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2655
Description  2.5A High Efficiency Synchronous Switching Regulator
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM2655 Datasheet(HTML) 8 Page - National Semiconductor (TI)

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Typical Performance Characteristics (Continued)
Operation
The
LM2655
is
a
constant
frequency
(300kHz),
current-mode PWM switcher that can be operated synchro-
nously 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 ef-
ficiencies compared to asynchronous operation because the
I
2R losses are reduced with the use of a MOSFET . Opera-
tion 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 in-
put source flows through the inductor and to the load. The
current from the high-side MOSFET is sensed and com-
pared 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 im-
proved. A feedforward from the input voltage is added to re-
duce 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
Current Limit vs Input Voltage
(Synchronous)
DS101284-13
Current Limit vs Input Voltage
(Asynchronous)
DS101284-14
Current Limit vs Junction Temperature
(V
IN = 5V, VOUT = 3.3V)
DS101284-15
Reference Voltage vs Junction Temperature
DS101284-16
www.national.com
8


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