APW7077/A
Function Description (Cont.)
Design Example(Cont.)
Determinetheoutput capacitancevaluefor thedesired
output ripple voltage:
Determine the average inductor current and peak in-
ductor current:
COUT=33uF
IL=1.38A
The ESR of the output capacitor is 0.05W. Therefore,
DIL=0.218A
Ipk=1.45A
a Tantalum capacitor with value of 33 uF to 47uF and
ESR of 0.05W can be used as the output capacitor.
However, according to experimental result, 220uF
output capacitorgivesbetter overalloperational stability
and smaller ripplevoltage.
Therefore, a 10 uH inductor with saturation current
larger than 1.73 A can be selected as the initial trial.
External Component Selection
External Switch Transistor
Diode Selection
AnenhancementN–channelMOSFETorabipolarNPN
transistor canbe used asthe external switch transistor.
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.
AnenhancementN–channel MOSFETcanbe selected
The output diode for a boost regulator must be cho-
sen correctly depending on the output voltage and the
output current. The diode must be rated for a reverse
voltageequal toorgreaterthantheoutputvoltageused.
The average current rating must be greater than the
maximum load current expected, and the peak cur-
rent rating must be greater than the peak inductor
current. During short circuit testing, or if short circuit
conditions are possible in the application, the diode
by the following guidelines:
·
Low ON–resistance, RDS(on).
current rating must exceed the switch current limit.
·
Low gate threshold voltage, VGS(th), typically
<1.5V, it is especially important for the low VOUT
device, like VOUT = 2.4V.
The diode is the largest source of loss in DC–DC
converters. The most importance parameters which
affect their efficiencyarethe forward voltagedrop, VF,
and the reverse recoverytime, trr.The forward voltage
drop creates a loss just by having a voltage across
the device while a current flowing through it. The re-
verse 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. Using
Schottky diodes with lower forward voltage drop will
decrease power dissipation and increase efficiency.
·
·
Rated continuous drain current, ID, should be
larger than the peak inductor current, i.e. ID> IPK.
Gate capacitance should be 1200 pF or less.
For bipolar NPN transistor, medium power transistor
with continuous collector current typically 1A to 5A
and VCE(sat) < 0.2 V should be employed. The driv-
ing capability is determined by the DC current gain,
HFE, of the transistor and the base resistor, Rb; and
Copyright ã ANPEC Electronics Corp.
15
www.anpec.com.tw
Rev. A.4 - Sep, 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
