NCP1402_06 Datasheet, PDF (15/18 Pages) ON Semiconductor – 200 mA, PFM Step−Up Micropower Switching Regulator

NCP1402
APPLICATIONS CIRCUIT INFORMATION
V_in
C1
10 mF
L1
D1
47 mH
CE
LX
1
5
OUT NCP1402
2
NC
GND
3
4
V_OUT
C2
68 mF
Figure 59. Typical Application Circuit
Step−up Converter Design Equations
NCP1402 step−up DC−DC converter designed to operate
in continuous conduction mode can be defined by:
Calculation
L
Equation
ǒ Ǔ v M
Vin2
VOUT IOmax
I_PK
(Vin
* Vs)ton
L
)
I
min
I_min
(ton
) toff)IO
toff
*
(Vin
* VS)ton
2L
t_off
(Vin * Vs)ton
(VOUT ) VF * Vin)
DQ
(IL * IO)toff
V_ripple
[
DQ
COUT
)
(IL
*
IO)ESR
*NOTES:
I_PK −
I_min −
I_O −
I_Omax −
I_L −
V_in −
V_OUT −
V_F −
V_S −
DQ −
V_ripple −
ESR −
M−
Peak inductor current
Minimum inductor current
Desired dc output current
Desired maximum dc output current
Average inductor current
Nominal operating dc input voltage
Desired dc output voltage
Diode forward voltage
Saturation voltage of the internal FET switch
Charge stores in the C_OUT during charging up
Output ripple voltage
Equivalent series resistance of the output capacitor
An empirical factor, when V_OUT ≥ 3.0 V,
M = 8 x 10^−6, otherwise M = 5.3 x 10^−6.
EXTERNAL COMPONENT SELECTION
Inductor
The NCP1402 is designed to work well with a 47 mH
inductor in most applications. 47 mH is a sufficiently low
value to allow the use of a small surface mount coil, but large
enough to maintain low ripple. Low inductance values
supply higher output current, but also increase the ripple and
reduce efficiency. Note that values below 27 mH is not
recommended due to NCP1402 switch limitations. Higher
inductor values reduce ripple and improve efficiency, but
also limit output current.
The inductor should have small DCR, usually less than 1
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.
Diode
The diode is the main 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, t_rr. 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 < 0.3 V
Small reverse leakage current
Fast reverse recovery time/ switching speed
Rated current larger than peak inductor current,
I_rated > I_PK
Reverse voltage larger than output voltage,
V_reverse > 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.
http://onsemi.com
15