NCP1400A
http://onsemi.com
13
APPLICATION CIRCUIT INFORMATION
Figure 41. Typical Step−Up Converter Application
1
3
GND
CE
2
OUT
NC
4
LX
5
NCP1400A
V
OUT
V
IN
C1
10 mF
L1
D1
C2
68 mF
22 mH
Step−up Converter Design Equations
General step−up DC−DC converter designed to operate in
discontinuous conduction mode can be defined by:
Calculation Equation
D
t
on
T
I
PK
V
in
t
on
L
I
O
(V
in
)
2
(t
on
)
2
f
2L(V
out
) V
F
* V
in
)
D − Duty cycle
I
PK
− Peak inductor current
I
O
− Desired dc output current
V
IN
− Nominal operating dc input voltage
V
OUT
− Desired dc output voltage
V
F
− Diode forward voltage
Assume saturation voltage of the internal FET switch is negligible.
External Component Selection
Inductor
Inductance values between 18 mH and 27 mH are the best
suitable values for NCP1400A. In general, smaller
inductance values can provide larger peak inductor current
and output current capability, and lower conversion
efficiency, and vice versa. Select an inductor with smallest
possible DCR, usually less than 1.0 W, to minimize loss. It
is necessary to choose an inductor with saturation current
greater than the peak current which the inductor will
encounter in the application. The inductor selected should be
able to handle the worst case peak inductor current without
saturation.
Diode
The diode is the largest source of loss in DC−DC
converters. The most importance parameters which affect
their efficiency are the forward voltage drop, V
F
, and the
reverse recovery time, trr. The forward voltage drop creates
a loss just by having a voltage across the device while a
current flowing through it. The reverse recovery time
generates a loss when the diode is reverse biased, and the
current appears to actually flow backwards through the
diode due to the minority carriers being swept from the P−N
junction. A Schottky diode with the following
characteristics is recommended:
Small forward voltage, V
F
t 0.3 V
Small reverse leakage current
Fast reverse recovery time/switching speed
Rated current larger than peak inductor current,
I
rated
u I
PK
Reverse voltage larger than output voltage,
V
reverse
u V
OUT
Input Capacitor
The input capacitor can stabilize the input voltage and
minimize peak current ripple from the source. The value of
the capacitor depends on the impedance of the input source
used. Small Equivalent Series Resistance (ESR) Tantalum
or ceramic capacitor with value of 10 mF should be suitable.
Output Capacitor
The output capacitor is used for sustaining the output
voltage when the internal MOSFET is switched on and
smoothing the ripple voltage. Low ESR capacitor should be
used to reduce output ripple voltage. In general, a 47 mF to
68 mF low ESR (0.15 W to 0.30 W) Tantalum capacitor
should be appropriate.
