SA57254-30GW Datasheet, PDF (13/18 Pages) NXP Semiconductors – CMOS switching regulator (PWM controlled)

Philips Semiconductors
CMOS switching regulator (PWM controlled)
Product data
SA57254-XX
Flyback converter
The SA57254-XX can also be used to create a flyback converter,
also known as an isolated boost converter. The advantage of a
flyback converter is that the input voltage can go higher or lower
than the output voltage without affecting the operation of the
converter. The only restrictions are the peak current flowing into the
switch pin (SW) and the breakdown voltage of the SW and feedback
(V_FB) pins.
One transformer can accommodate a variety of output voltages in
different applications, because the circuit will change the on and
off–times to provide the desired output voltage.
The output voltage of the flyback can be changed by using a
SA57254-XX with the desired output voltage, with no other changes
to the circuit.
Selecting the components
It is best to operate the transformer in the continuous-mode where
the highest expected peak primary current is below the maximum
current rating of the SA57254-XX switch.
Because the SA57254-XX is a peak current-limitied IC, begin with a
peak current equal to or less than the maximum current rating of the
part (0.3 A). A reasonable value of the primary inductance can be
found in Equation (9).
L_pri t 5V_IN(min)
T_on
I_peak
Eqn. (9)
Where:
I_peak is 0.3 A or less.
T_on is the maximum expected on-time of the switch (≈10 µs).
V_IN(min) is the lowest expected input voltage (V).
Then select an off-the-shelf transformer such as the Coiltronics
CTX100–1P, a 1:1 turns ratio transformer that has a primary
inductance of 100 µH. It does not reach saturation until the primary
current reaches 440 mA, which is above the expected peak current
of the flyback converter. The 1:1 turns ratio should work for output
voltages from 0.8 to 2 times the highest input voltage, and produce
the output voltage set by the SA57254-XX. The only other restriction
is that the input voltage plus the output voltage must be less than
the breakdown voltage of the SA57254-XX (9 V).
Use Equation (8) to determine the minimum value for the input
capacitor. A 0.1 V drop is desired across this capacitor.
C_IN
u
(0.3A) (10ms)
0.1V
+
30mF
A 47 µF at 6 V tantalum capacitor would be suitable.
For the design example, the output voltage will be +3.3 V with a
maximum output current of 50 mA. The input voltage can vary
between +1.8 V and 4.0 V. The design can be seen in Figure 27,
and the expected waveforms can be seen in Figure 28.
V_BATT
47 µF
@ 10 V
COILTRONICS
CTX100-1P
47 µF
@ 10 V
V_OUT
3.3 V
@ 0.1 A
SW
FB
SA57254-33
GND
Figure 27. Flyback converter circuit.
SL01505
V_IN + V_OUT
(1:1 TRANSFORMER)
V_IN
+V_OUT
GROUND (0 V)
–V_IN
I_SW(peak)
I_diode(peak)
V_IN
I_peak = I_diode
(1:1 TRANSFORMER)
2003 Nov 11
SL01468
Figure 28. Flyback converter waveforms.
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