SS6638-27G 查看數據表（PDF） - Silicon Standard Corp.
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SS6638-27G Datasheet PDF : 19 Pages
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VEXT
IIN
IPK
IV
ISW
ID
VSW
IOUT
t
Continuous Conduction Mode
At the boundary between continuous and
discontinuous mode, output current (IOB) is
determined by
IOB = VIN 1 VIN TON(1− x)
VOUT + VD 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
frequency (Fsw) is
Fsw = 2(L)(VOUT + VD − VIN)(IOUT) (1 + x)
VIN2 × TON2
In the continuous mode, the switching frequency
is
SS6638G
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
element, the inductor must fulfill three
requirements. First, the inductance must be low
enough for the inductor to store adequate energy
under the worst-case condition of minimum input
voltage and switch ON time. Second, the
inductance must also be high enough so the
maximum current rating of the SS6638 and 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
before an 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 (Simple losses equation),
L
where x=(RON+RS)TON/L
8/21/2005 Rev.2.3
www.SiliconStandard.com
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