SS6642-27G 查看數據表（PDF） - Silicon Standard Corp.
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SS6642-27G Datasheet PDF : 22 Pages
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VEXT
IIN
IPK
ISW
ID
IOUT
VSW
t
Continuous Conduction Mode
Continuous Conduction Mode
At the boundary between continuous and dis-
continuous modes, output current (IOB) is deter-
mined by
IOB = VIN * 1 * VIN * TON * (1 − x)
VOUT 2 L
where Vd is the diode drop,
x = (RON + RS) * TON
L
RON= Switch turn-on resistance, RS= Inductor
DC resistance
TON = Switch ON time
In the discontinuous mode, the switching fre-
quency (Fsw) is
Fsw = 2 * (L) * (VOUT + VD − VIN) * (IOUT) (1 + x)
VIN2 × TON2
SS6642G
In the continuous mode, the switching fre-
quency is
( ) fSW = 1 VOUT + VD − VIN
TON (VOUT + VD − VSW)
* [1 + x ( VIN − VSW )]
2 VOUT + VD − VSW
≅ 1 VOUT + VD − VIN
TON VOUT + VD − VSW
where Vsw = switch drop and is proportional to
output current.
Inductor Selection
To operate as an efficient energy transfer ele-
ment, the inductor must fulfill three require-
ments. First, the inductance must be low
enough for the inductor to store adequate en-
ergy under the worst-case condition of mini-
mum input voltage and switch ON time. Second,
the inductance must also be high enough so that
the maximum current rating of the SS6642 and
the inductor are not exceeded at the other worst-
case condition of maximum input voltage and ON
time. Lastly, the inductor must have sufficiently
low DC resistance so excessive power is not lost
as heat in the windings. Unfortunately, this is
inversely related to physical size.
Minimum and maximum input voltage, output
voltage and output current must be established
in advance and then the inductor can be selected.
In discontinuous mode operation, at the end of
the switch ON time, peak current and energy in
the inductor build according to
IPK = VIN * 1 − exp(− RON + Rs * TON)
RON + Rs
L
≅ VIN * (TON) * 1 − x
L
2
≅ VIN TON
L
(simple loss equation),
where x = (RON + RS) * TON
L
1/15/2005 Rev.2.10
www.SiliconStandard.com
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