lm27222sd National Semiconductor Corporation, lm27222sd Datasheet - Page 8

• Manufacturer Part Number: lm27222sd
• Description: High-speed 4.5a Synchronous Mosfet Driver
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
POWER DISSIPATION
The power dissipated in the driver IC when switching syn-
chronously can be calculated as follows:
where f
V
Q
high-side MOSFET(s)
Q
low-side MOSFET(s)
R
high-side MOSFET(s)
R
low-side MOSFET(S)
R
R
R
R
PC BOARD LAYOUT GUIDELINES
1. Place the driver as close to the MOSFETs as possible.
2. HG, SW, LG, GND: Run short, thick traces between the
3. Driver V
CC
G_H
G_L
H_pu
H_pd
L_pu
L_pd
G_H
G_L
driver and the MOSFETs. To minimize parasitics, the
traces for HG and SW should run parallel and close to
each other. The same is true for LG and GND.
V
= voltage at the V
CC
= total gate charge of the (parallel combination of the)
= gate resistance of the (parallel combination of the)
= total gate charge of the (parallel combination of the)
= gate resistance of the (parallel combination of the)
= pull-up R
= pull-down R
= pull-up R
= pull-down R
SW
and GND pins.
= switching frequency
CC
: Place the decoupling capacitor close to the
DS_ON
DS_ON
DS_ON
DS_ON
CC
of the low-side driver
of the high-side driver
pin,
of the low-side driver
of the high-side driver
(Continued)
8
4. The high-current loop between the high-side and low-
5. There should be enough copper area near the MOS-
TYPICAL APPLICATION CIRCUIT DESCRIPITON
The Application Example on the following page shows the
LM27222 being used with National’s LM27212, a 2-phase
hysteretic current mode controller. Although this circuit is
capable of operating from 5V to 28V, the components are
optimized for an input voltage range of 9V to 28V. The
high-side FET is selected for low gate charge to reduce
switching losses. For low duty cycles, the average current
through the high-side FET is relatively small and thus we
trade off higher conduction losses for lower switching losses.
The low-side FET is selected solely on R
conduction losses. If the input voltage range were 4V to 6V,
the MOSFET selection should be changed. First, much lower
voltage FETs can be used, and secondly, high-side FET
R
losses. Of course with a lower input voltage, the input ca-
pacitor voltage rating can be reduced and the inductor value
can be reduced as well. For a 4V to 6V application, the
inductor can be reduced to 200nH to 300nH. The switching
frequency of the LM27212 is determined by the allowed
ripple current in the inductor. This circuit is set for approxi-
mately 300kHz. At lower input voltages, higher frequencies
are possible without suffering a significant efficiency loss.
Although the LM27222 can support operating frequencies up
to 2MHz in many applications, the LM27212 should be lim-
ited to about 1MHz. The control architecture of the LM27212
and the low propagation times of the LM27222 potentially
gives this solution the fastest transient response in the
industry.
DS_ON
side MOSFETs and the input capacitors should be as
small as possible.
FETs and the inductor for heat dissipation. Vias may
also be added to carry the heat to other layers.
becomes a larger loss factor than the switching
DS_ON
to minimize