NCP1450A
http://onsemi.com
19
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
D
, 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 a value of 10 F should be
suitable.
Output Capacitor
The output capacitor is used for sustaining the output
voltage when the external MOSFET or bipolar transistor is
switched on and smoothing the ripple voltage. Low ESR
capacitor should be used to reduce output ripple voltage. In
general, a 100 F to 220 F low ESR (0.10 to 0.30 )
Tantalum capacitor should be appropriate.
External Switch Transistor
An enhancement N−channel MOSFET or a bipolar NPN
transistor can be used as the external switch transistor.
For enhancement N−channel MOSFET, since
enhancement MOSFET is a voltage driven device, it is a
more efficient switch than a BJT transistor. However, the
MOSFET requires a higher voltage to turn on as compared
with BJT transistors. An enhancement N−channel MOSFET
can be selected by the following guidelines:
1. Low ON−resistance, R
DS(on)
, typically < 0.1 .
2. Low gate threshold voltage, V
GS(th)
, must be <
V
OUT
, typically < 1.5 V, it is especially important
for the low V
OUT
device, like V
OUT
= 1.9 V.
3. Rated continuous drain current, I
D
, should be
larger than the peak inductor current, i.e. I
D
> I
PK
.
4. Gate capacitance should be 1200 pF or less.
For bipolar NPN transistor, medium power transistor with
continuous collector current typically 1 A to 5 A and V
CE(sat)
< 0.2 V should be employed. The driving capability is
determined by the DC current gain, H
FE
, of the transistor and
the base resistor, Rb; and the controller’s EXT pin must be
able to supply the necessary driving current.
Rb can be calculated by the following equation:
Rb +
V
OUT *
0.7
Ib
*
0.4
|
I
EXTH
|
Ib +
I
PK
H
FE
Since the pulse current flows through the transistor, the
exact Rb value should be finely tuned by the experiment.
Generally, a small Rb value can increase the output current
capability, but the efficiency will decrease due to more
energy is used to drive the transistor.
Moreover, a speed−up capacitor, Cb, should be connected
in parallel with Rb to reduce switching loss and improve
efficiency. Cb can be calculated by the equation below:
Cb v
1
2 Rb f
OSC
0.7
It is due to the variation in the characteristics of the
transistor used. The calculated value should be used as the
initial test value and the optimized value should be obtained
by the experiment.
External Component Reference Data
Device V
OUT
Inductor
Model
Inductor
Value
External
Transistor
Diode
Output
Capacitor
NCP1450ASN19T1 1.9 V CD54
12 H
NTGS3446T1 MBRM110L
220 F
NCP1450ASN30T1 3.0 V CD54
10 H
NTGS3446T1 MBRM110L
220 F
NCP1450ASN50T1 5.0 V CD54
10 H
NTGS3446T1 MBRM110L
220 F
NCP1450ASN19T1 1.9 V CD54
12 H
MMJT9410 MBRM110L
220 F
NCP1450ASN30T1 3.0 V CD54
10 H
MMJT9410 MBRM110L
220 F
NCP1450ASN50T1 5.0 V CD54
10 H
MMJT9410 MBRM110L
220 F
