AIC1639
3-PIN ONE-CELL STEP-UP DC/DC CONVERTER
SPEC NO: DS-1639-00 05/26/00
V
EXT
≅
1
Vout
+
Vd
−
Vin
Ton
Vout
+
Vd
−
Vsw
I
IN
where Vsw = switch drop and proportion to output cur-
rent.
I
PK
INDUCTOR SELECTION
To operate as an efficient energy transfer element, the
inductor must fulfill three requirement. First, the induc-
tance must be low enough for the inductor to store
adquate energy under the worst case condition of
minimum input voltage and switch ON time. Second,
the inductance must also be high enough so maxi-
mum current rating of AIC1639 and inductor are not
exceed at the other worst case cindition 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. But unfo-
trunately this is inversely related to physical size.
t
I
SW
I
D
I
OUT
V
SW
Continuous Conduction Mode
Minimum and Maximum input voltage, output voltage
and output current must be established before and in-
cuctor can be selected.
In discontinuous mode operation, at the end of the
switch ON time, peak current and energy in the induc-
tor build according to
At the boundry between continuous and discontinu-
ous mode, output current (IOB) is determined by
Vin
1 Vin
I
OB
=
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 re-
sistance
Ton = Switch ON time
In the discontinuous mode, the switching frequency
(fsw) is
fsw =
2(L)(Vout
+
Vd
−
Vin)(Iout)
Vin
2
×
Ton
2
(1
+
x )
Ron
+
Rs
Vin
I
PK
=
Ton)
1
−
exp(
−
L
Ron
+
Rs
x
Vin
≅
(
Ton
)
1
−
2
L
≅
Vin
Ton
L
(simple lossless equation),
where x=(Ron+Rs)Ton/L
E
L
=
1
L
×
Ipk
2
2
Power required from the inductor per cycle must be
equal or greater than
P
L
/f
SW
=
(Vout
+
Vd
−
Vin)(Iout) (
1
)
fsw
In the continuous mode, the switching frequency is
in order for the converter to regulate the output.
When loading is over IOB, PFM controller operates in
continuous mode. Inductor peak current can be derived
from
fsw=
1
(
Vout
+
Vd
−
Vin
)
x
Vin
−
Vsw
[1
+
(
)]
Ton (Vout
+
Vd
−
Vsw)
2 Vout
+
Vd
−
Vsw
ANALOG INTEGRATIONS CORPORATION
4F, 9 Industry E. 9th Road, Science-based Industrial Park , Hsinchu, Taiwan, R.O.C.
x
Vout
+
Vd
−
Vsw x
Vin
−
Vsw
I
PK
=
−
Iout
+
Ton
1
−
Vin
−
Vsw
2
2L
2
www.analog.com.tw
TEL: (8863)577-2500 FAX:(8863)577-2510
9
AIC [ ANALOG INTERGRATIONS CORPORATION ]
