Datasheet
Design Procedure (Continued)
ment. A reasonable value is setting the ripple current to be
30% of the DC output current. Since the ripple current in-
creases 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 resistance 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
OUT
is primarily determined by the maxi-
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
entering the sleep mode at full load:
V
RIPPLE
<
20mV * V
OUT
/V
FB
TABLE 1. MOSFET Manufacturers
Manufacturer Model Number Package Type www Address Phone Fax
Fairchild
Semiconductor
FDC653N SuperSOT-6 www.fairchildsemi.com 888-522-5372 207-761-6020
General
Semiconductor
GF4420 SO-8 www.gensemi.com 631-847-3000 631-847-3236
International
Rectifier
IRF7807 SO-8 www.irf.com 310-322-3331 310-322-3332
Vishay Siliconix Si4812DY SO-8 www.vishay.com 800-554-5565 408-567-8995
Si4874DY SO-8
Zetex ZXM64N03X SO-8 www.zetex.com (44) 161-622-4422 (44) 161-622-4420
LOW-SIDE MOSFET SELECTION
When operating in synchronous mode, special attention
should be given to the selection of the low-side MOSFET.
Besides choosing a MOSFET with minimal size and on
resistance, it is critical that the MOSFET meet certain rise
and fall time specifications. A 30ns deadtime between the
low-side and high-side MOSFET switching transitions is pro-
grammed into the LM2655, as shown in Figure 1. The pre-
vent shoot-through current, the low-side MOSFET must turn
off before the high-side MOSFET turns on. Hence, the low-
side MOSFET has 30ns to turn off from the time the low-side
driver goes low. The fall time of the low-side MOSFET is
governed by the equation:
I
C
=C
IN
*dV
C
/dt.
where I
C
is the LDR sink current capability, C
IN
is the equiva-
lent capacitance seen at the LDR pin, and V
C
is the gate-to-
source voltage of the MOSFET. I
C
is limited by the low-side
driver of the LM2655, but C
IN
is fixed by the MOSFET.
Therefore, it is important that the chosen MOSFET has a
suitable C
IN
so that the LM2655 will be able to turn it off
10128421
FIGURE 1. Low-side/high-side driver timing diagram.
LM2655
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