Fast PFET Buck Controller
2) Switching Frequency and On and Off Time
Calculations
Given that f
SW
= 200 kHz and D
MAX
= 0.80
LM51031
impedance aluminum are less expensive.
of suppliers and are the best choice for surface mount
Solid tantalum chip capacitors are available from a numb
of suppliers and are the best choice for surface mount
applications.
The output capacitor limits the output ripple voltage. The
LM51031 needs a maximum of 20 mV of output ripple for
the feedback comparator to change state. If we assume
that all the inductor ripple current flows through the output
capacitor and that it is an ideal capacitor (i.e. zero ESR),
3) Oscillator Capacitor Selection
The switching frequency is set by COSC, whose
value is given by:
the minimum capacitance needed to limit the output ripple
to 50 mV peak−to−peak is given by:
The minimum ESR needed to limit the output voltage
ripple to 50 mV peak−to−peak is:
4) Inductor Selection
The inductor value is chosen for continuous mode
operation down to 0.3 Amps.
The ripple current ∆I = 2 × I
OUTmin
= 2 × 0.3 A
= 0.6 A
The output capacitor should be chosen so that its ESR is
less than 83 mΩ.
During the minimum off time, the ripple current is 0.4 A
and the output voltage ripple will be:
This is the minimum value of inductor to keep the
ripple current < 0.6 A during normal operation.
A smaller inductor will result in larger ripple
current. Ripple current at a minimum off time is:
6) VFB Divider
The input bias current to the comparator is 4.0 uA. The
resistor divider current should be considerably higher than
this to ensure that there is sufficient bias current. If we
The core must not saturate with the maximum
expected current, here given by:
choose the divider current to be at least 250 times the bia
current this permits a divider current of 1.0 mA and
simplifies the calculations.
5) Output Capacitor
The output capacitor and the inductor form a low
pass filter. The output capacitor should have a low
ESL and ESR. Low impedance aluminum electrolytic,
tantalum or organic semiconductor capacitors are
a good choice for an output capacitor. Low
Let R2 = 1.0 K
Rearranging the divider equation gives:
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