LM51031D HTC [HTC Korea TAEJIN Technology Co.], LM51031D Datasheet - Page 7

• Manufacturer Part Number: LM51031D
• Description: Fast PFET Buck Controller
• Manufacturer: HTC [HTC Korea TAEJIN Technology Co.]

2) Switching Frequency and On and Off Time
Calculations
3) Oscillator Capacitor Selection
value is given by:
4) Inductor Selection
operation down to 0.3 Amps.
= 0.6 A
ripple current < 0.6 A during normal operation.
current. Ripple current at a minimum off time is:
expected current, here given by:
5) Output Capacitor
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
Fast PFET Buck Controller
Given that f
The switching frequency is set by COSC, whose
The inductor value is chosen for continuous mode
The ripple current ∆I = 2 × I
This is the minimum value of inductor to keep the
A smaller inductor will result in larger ripple
The core must not saturate with the maximum
The output capacitor and the inductor form a low
SW
= 200 kHz and D
OUTmin
MAX
= 2 × 0.3 A
= 0.80
− 7 −
to 50 mV peak−to−peak is given by:
less than 83 mΩ.
choose the divider current to be at least 250 times the bia
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.
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),
the minimum capacitance needed to limit the output ripple
ripple to 50 mV peak−to−peak is:
The output capacitor should be chosen so that its ESR is
During the minimum off time, the ripple current is 0.4 A
and the output voltage ripple will be:
6) VFB Divider
resistor divider current should be considerably higher than
this to ensure that there is sufficient bias current. If we
current this permits a divider current of 1.0 mA and
simplifies the calculations.
The output capacitor limits the output ripple voltage. The
The minimum ESR needed to limit the output voltage
The input bias current to the comparator is 4.0 uA. The
Rearranging the divider equation gives:
Let R2 = 1.0 K
LM51031
HTC