AIC1642 Datasheet, PDF (11/14 Pages) Analog Intergrations Corporation – 3-Pin One-Cell Step-Up DC/DC Converter

V_EXT
I_IN
I_PK
I_SW
I_D
I_OUT
V_SW
t
Continuous Conduction Mode
Continuous Conduction Mode
At the boundary between continuous and dis-
continuous mode, output current (I_OB) 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
R_ON= Switch turn on resistance, R_S= Inductor
DC resistance
T_ON = Switch ON time
In the discontinuous mode, the switching fre-
quency (Fsw) is
Fsw = ^{2 * (L) * (VOUT + VD − VIN) * (IOUT)} (1+ x)
VIN ² × TON ²
AIC1642
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 proportion to out-
put 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 minimum
input voltage and switch ON time. Second, the
inductance must also be high enough so maxi-
mum current rating of AIC1642 and inductor are
not exceed at the other worst case condition of
maximum input voltage and ON time. Lastly, the
inductor must have sufficiently low DC resis-
tance so excessive power is not lost as heat in
the windings. But 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 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
 ²ï£¸
≅ ^VIN TON
L
(simple loss equation),
where x = (RON + RS) * ^TON
L
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