
AIC1641
LOW VOLTAGE STEP-UP DC/DC CONVERTER
SPEC NO: DS-1641-00 05/04/00
ANALOG INTEGRATIONS CORPORATION
4F, 9 Industry E. 9th Road, Science-based Industrial Park , Hsinchu, Taiwan, R.O.C. TEL: (8863)577-2500 FAX:(8863)577-2510
www.analog.com.tw
9
I
PK
I
IN
V
EXT
I
SW
I
OUT
V
SW
I
D
t
Continuous Conduction Mode
t the boundry between continuous and discontinuous
mode, output current (IOB) is determined by
I
OB
=
(
)
x
1
T
L
V
2
1
V
V
V
ON
IN
D
OUT
IN
+
where Vd is the diode drop,
x = (R
ON
+Rs)T
ON
/L.
Ron= Switch turn on resistance, Rs= Inductor DC re-
sistance
T
ON
= Switch ON time
In the discontinuous mode, the switching frequency
(fsw) is
F
SW
=
x
(1
T
V
)
)(I
V
V
×
2(L)(V
2
ON
2
IN
OUT
IN
D
OUT
+
+
In the continuous mode, the switching frequency is
fsw=
(
)
)
)]
V
V
V
V
V
(
2
x
[1
V
V
(V
V
V
V
T
1
SW
D
OUT
SW
IN
+
SW
D
OUT
IN
D
OUT
ON
+
+
+
+
+
SW
D
OUT
IN
D
OUT
ON
V
V
V
V
V
V
T
1
where Vsw = switch drop and proportion to output
current.
INDUCTOR SELECTION
To operate as an efficient energy transfer element,
the inductor must fulfill three requirement. First, the
inductance must be low enough for the inductor to
store adquate energy under the worst case condition
of minimum input voltage and switch ON time. Sec-
ond, the inductance must also be high enough so
maximum current rating of AIC1641 and inductor are
not exceed at the other worst case cindition of
maximum input voltage and ON time. Lastly, the in-
ductor must have sufficiently low DC resistance so
excessive power is not lost as heat in the windings.
But unfotrunately this is inversely related to physical
size.
Minimum and Maximum input voltage, output volt-
age and output current must be established before
and incuctor can be selected.
In discontinuous mode operation, at the end of the
switch ON time, peak current and energy in the in-
ductor build according to
+
+
=
)
T
L
Rs
R
exp(
1
Rs
R
V
I
ON
ON
ON
IN
PK
(
)
2
x
1
T
L
V
ON
IN
ON
IN
T
L
V
(simple lossless equation),
where x=(R
ON
+Rs)T
ON
/L
PK
I
L
2
1
×
Power required from the inductor per cycle must be
equal or greater than
)
fsw
1
)(
)(I
V
V
(V
/f
P
OUT
IN
D
OUT
SW
L
+
=
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 de-
rived from