Philips Semiconductors
Product data
CMOS switching regulator (PWM controlled)
SA57250-XX
Determining the value of the boost inductor
The minimum value of the output capacitor can be estimated by
Equation (7).
The precise value of the boost inductor is not critical to the operation
of the SA57250-XX. The value of the boost inductor should be
calculated to provide continuous-mode operation over most of its
operating range. The converter may enter the discontinuous-mode
when the output load current falls to less than about 20 percent of
the full-load current.
(IOUT(max)) (Toff
)
Eqn. (7)
COUT
Where:
u
Vripple(p*p)
I
is the average value of the output load current (A).
OUT
At low input voltages, the time required to store the needed energy
lengthens, but the time needed to empty the inductor’s core of its
energy shrinks. Conversely, at high input voltages, the time needed
to store the energy shrinks while the time needed to empty the core
increases. See Equations (1) and (3). At the extremes of these
conditions, the converter will fall out of regulation, that is the output
voltage will begin to fall, because the time needed for either storing
or emptying the stored inductor energy is too short to support the
output load current.
T
V
is the nominal off–time of the power switch (sec) [ 10 µs].
off
is the desired amount of ripple voltage (V ).
p–p
ripple
Finding the value of the input capacitor is done by Equation (8).
(Ipeak) (Ton
Vdrop
)
Eqn. (8)
CIN
Where:
u
I
is the expected maximum peak current of the switch (A).
peak
T
V
is the on-time of the switch (sec) [ 10 µs].
on
Equation (4) determines the nominal value of the inductance.
is the desired amount of voltage drop across the capacitor
drop
VIN(min) Ton
(V ).
p–p
Eqn. (4)
L0 ^
Ipeak
These calculations should produce a good estimate of the needed
values of the input and output capacitors to yield the desired ripple
voltages.
Where:
V
is the lowest expected input operating voltage (V).
IN(min)
T
is about 10 µs or one-half the switching period (s).
is the maximum peak current for the SA57250-XX (0.3 A).
on
Selecting the output rectifier
I
peak
The output rectifier (D) is critical to the efficiency and low-noise
operation of the boost converter. The majority of the loss within the
supply will be caused by the output rectifier. Three parameters are
important in the rectifier’s operation within a boost-mode supply.
These are defined below.
This is an estimated inductor value and you can select an
inductance value slightly higher or lower with little effect on the
converter’s operation. If the design falls out of regulation within the
desired operating range, reduce the inductance value, but by no
more than 30 percent.
Forward voltage drop (V )—This is the voltage across the rectifier
f
when a forward current is flowing through the rectifier. A P-N
ultra-fast diode exhibits a 0.7 – 1.4 volt drop, and this drop is
relatively fixed over the range of forward currents. A Schottky diode
exhibits a 0.3 – 0.6 volt drop and appears more resistive during the
forward conduction periods. That is, its forward voltage drop
increases with increasing currents. You can gain an advantage by
purposely over-rating the current rating of a Schottky rectifier.
Determining the minimum value of the capacitors
The input and output capacitors experience the current waveforms
seen in Figures 26 and 27. The peak currents can be typically
between 3 to 6 times the average currents flowing into the input and
from the output. This makes the choice of capacitor an issue of how
much ripple voltage can be tolerated on the capacitor’s terminals
and how much heating the capacitor can tolerate. At the power
levels produced by the SA57250-XX heating is not a major issue.
Reverse recovery time (T )—This is an issue when the boost
rr
supply is operating in the continuous-mode. T is the amount of time
rr
The Equivalent Series Resistance (ESR) of the capacitor, the
resistance that appears between its terminals, and the actual
capacitance causes heat to be generated within the case whenever
there is current entering or exiting the capacitor. ESR also adds to
the apparent voltage drop across the capacitor. The heat that is
generated can be approximated by Equation (5).
required for the rectifier to assume an open circuit when a forward
current is flowing and a reverse voltage is then placed across its
terminals. P-N ultra-fast rectifiers typically have a 25–40 ns reverse
recovery time. Schottky rectifiers have a very short or no reverse
recovery time.
Forward recovery time (T )—This is the amount of time before a
fr
Eqn. (5)
PD(in watts) ^ (1.8Iav) 2(RESR
)
rectifier begins conducting forward current after a forward voltage is
placed across its terminals. This parameter is not always well
specified by the rectifier manufacturers. It causes a spike to appear
when the power switch turns off. This particular point in its operation
causes the most radiated noise. Several rectifiers may have to be
evaluated for the prototype. After the final output rectifier selection is
made, if the spike is still causing a problem a small passive snubber
can be placed across the rectifier.
ESR’s effect on the capacitor voltage is given by Equation (6).
DVC ^ Ipeak(RESR
)
(expressed as V
)
Eqn. (6)
p-p
A ceramic capacitor would typically be used in this application if the
required value is less than 1 – 10 µF, or a tantalum capacitor for
required values of 10 µF and above. Lower cost aluminum
electrolytic capacitors can be used, but you should confirm that the
higher ESRs typically exhibited by these capacitors does not cause
a problem.
For this boost application, the best choice of output rectifier is a low
forward drop, 0.5 – 1 ampere, 20 volt Schottky rectifier such as the
Philips part number BAT120A.
12
2003 Nov 12
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