Fast PFET Buck Controller
LM51031
7) Divider Bypass Capacitor CRR
Since the feedback resistors divide the output
voltage by a factor of 4.0, i.e. 5.0 V/1.25 V = 4.0,
it follows that the output ripple is also divided by
four. This would require that the output ripple be at
comparator. We use a capacitor CRR to act as an
least 60 mV (4.0 × 15 mV) to trip the feedback AC
short. The ripple voltage frequency is equal to the
switching frequency so we choose CRR = 1.0 nF.
The fast discharge time occurs when a fault is first
detected. The CS capacitor is discharged from 2.5 V to 2
where IFastDischarge is 66 uA typical.
8) Soft−Start and Fault Timing Capacitor CS
CS performs several important functions. First it
provides a delay time for load transients so that the
IC does not enter a fault mode every time the load
changes abruptly. Secondly it disables the fault
circuitry during startup, it also provides Soft−Start
by clamping the reference voltage during startup,
allowing it to rise slowly, and, finally it controls the
hiccup short circuit protection circuitry. This reduces
the duty cycle to approximately 0.035 during short
circuit conditions. An important consideration in
calculating CS is that it’s voltage does not reach
2.5 V (the voltage at which the fault detect circuitry
is enabled) before VFB reaches 1.15 V otherwise
the power supply will never start. If the VFB pin
reaches 1.15 V, the fault timing comparator will
discharge CS and the supply will not start. For the
VFB voltage to reach 1.15 V the output voltage must
be at least 4 × 1.15 = 4.6 V. If we choose an
arbitrary startup time of 900 s, the value of CS is:
The recharge time is the time for CS to charge from 1.5
to 2.5 V.
where ICharge is 264 uA typical.
The fault time is given by:
For this circuit
A larger value of CS will increase the fault time out time
but will also increase the Soft−Start time.
9) Input Capacitor
The input capacitor reduces the peak currents drawn
from the input supply and reduces the noise and ripple
voltage on the VCC and VC pins. This capacitor must also
ensure that the VCC remains above the UVLO voltage in
the event of an output short circuit. A low ESR capacitor
output short circuit. A low ESR capacitor of at least 100 u
is good. A ceramic surface mount capacitor should also
be connected between VCC and ground to filter high
frequency noise.
The fault time is the sum of the slow discharge time
the fast discharge time and the recharge time. It is
dominated by the slow discharge time.
The first parameter is the slow discharge time, it is
the time for the CS capacitor to discharge from 2.4 V
to 1.5 V and is given by:
10) MOSFET Selection
The LM51031 drives a P−Channel MOSFET. The VGATE
pin swings from GND to VC. The type of P−Ch FET used
depends on the operating conditions but for input voltage
below 7.0 V a logic level FET should be used.
where IDischarge is 6.0 uA typical.
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