Application Information: continued
The phase lead provided by this zero ensures that the loop
where:
N = transformer turns ratio primary over secondary.
has at least 45° phase margin at the cross-over frequency.
Therefore, this zero shall be placed close to the pole gener-
ated in the power stage which can be identified at fre-
quency:
When the power switch turns off, there exists a voltage
spike superimposed on top of the steady–state voltage.
Usually this voltage spike is caused by transformer leakage
inductance charging stray capacitance between the VSW
and PGnd pins. To prevent the voltage at the VSW pin from
exceeding the maximum rating, a transient voltage sup-
pressor in series with a diode is paralleled with the pri-
mary windings. Another method of clamping switch volt-
age is to connect a transient voltage suppressor between
the VSW pin and ground.
1
fP =
2πCORLOAD
where:
CO = equivalent output capacitance of the error amplifier
≈ 120pF;
RLOAD= load resistance.
The second pole, fP2, located at high frequency range, can
be placed at the output filter’s ESR zero or half the switch-
ing frequency to cut down the switching noises. The fre-
quency of this pole is determined by the value of C2 and
R1:
Magnetic Component Selection
When choosing a magnetic component, one must consider
factors such as peak current, core and ferrite material, out-
put voltage ripple, EMI, temperature range, physical size
and cost. In boost circuits, the average inductor current is
output current times voltage gain (VOUT/VCC), assuming
100% energy transfer efficiency. In continuous conduction
mode, inductor ripple current is
1
fP2
=
2πC2R1
One simple method to the ensure adequate phase margin
is to design the frequency response tilted at −20dB/dec
slope all the way to unity-gain crossover. The cross-over
frequency shall be selected at the midpoint between fZ1 and
VCC(VOUT – VCC
)
IRIPPLE
=
(f)(L)(VOUT
)
fP2 where the phase margin is maximized.
where:
f = 270kHz
The peak inductor current is equal to average current plus
half of the ripple current, which should not cause inductor
saturation. The above equation can also be referenced on
selecting the value of the inductor based on the tolerance
of the ripple current in the circuits. Small ripple current
provides the benefits of small input capacitors and greater
output current capability. A core geometry like a rod or
barrel is prone to generating high magnetic field radiation,
but is relatively cheap and small. Other core geometry,
such as a toroid, provides a closed magnetic loop to pre-
vent EMI.
fp1
fz1
fp2
Frequency (LOG)
Input Capacitor Selection
Figure 6. Bode plot of the compensation network shown in Figure 6.
VSW Voltage Limit
In the boost circuit the VSW pin maximum voltage is set by
the maximum output voltage plus output diode forward
voltage. The diode forward voltage is typically 0.5V for
Schottky diodes and 0.8V for ultrafast recovery diodes
VSW(MAX) = VOUT(MAX) + VF
where:
1: Input voltage VCC ripple (AC coupled)
2: Input Current IIN
3: Inductor Current IL
VF = output diode forward voltage.
In the flyback circuit, the peak VSW voltage is governed by:
Figure 7a: Boost input voltage and current ripple waveforms.
VSW(MAX) = VCC(MAX) + (VOUT + VF) × N
9
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