Datasheet

C1 =
I x t
ON
'V
0.15A x 2.47 Ps
2.0V
=
= 0.185 PF
LM5009
SNVS402G –FEBRUARY 2006–REVISED FEBRUARY 2013 www.ti.com
C2 should generally be no smaller than 3.3 µF. Typically, its value is 10 µF to 20 µF, with the optimum value
determined by the load. If the load current is fairly constant, a small value suffices for C2. If the load current
includes significant transients, a larger value is necessary. For each application, experimentation is needed to
determine the optimum values for R3 and C2.
C) Ripple Reduction: The ripple amplitude at V
OUT1
can be reduced by reducing R3, and adding a capacitor
across R1 so as to transfer the ripple at V
OUT1
directly to the FB pin, without attenuation. The new value of R3 is
calculated from:
R3 = 25 mV/I
OR(min)
(8)
where I
OR(min)
is the minimum ripple current amplitude - 33 mAp-p in this example. The added capacitor's value is
calculated from:
C = T
ON(max)
/(R1 // R2) (9)
where T
ON(max)
is the maximum on-time (at minimum Vin). The selected capacitor should be larger than the value
calculated above.
R
CL
: When a current limit condition is detected, the minimum off-time set by this resistor must be greater than the
maximum normal off-time which occurs at maximum Vin. Using Equation 4, the minimum on-time is 0.329 µs,
yielding a maximum off-time of 2.63 µs. This is increased by 82 ns (to 2.72 µs) due to a ±25% tolerance of the
on-time. This value is then increased to allow for:
The response time of the current limit detection loop (400ns).
The off-time determined by Equation 5 has a ±25% tolerance:
t
OFFCL(MIN)
= (2.72 µs x 1.25) + 0.4 µs= 3.8 µs (10)
Using Equation 5, R
CL
calculates to 167 kΩ (at V
FB
= 2.5V). The closest standard value is 169 kΩ.
D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time
determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage
drop is significant in the event the output is short-circuited as it is only this diode’s voltage which forces the
inductor current to reduce during the forced off-time. For this reason, a higher voltage is better, although that
affects efficiency. A good choice is an ultrafast power or Schottky diode with a reverse recovery time of ≊30 ns,
and a forward voltage drop of ≊0.7V. Other types of diodes may have a lower forward voltage drop, but may
have longer recovery times, or greater reverse leakage. D1’s reverse voltage rating must be at least as great as
the maximum Vin, and its current rating be greater than the maximum current limit threshold (370 mA).
C1: This capacitor’s purpose is to supply most of the switch current during the on-time, and limit the voltage
ripple at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero.
At maximum load current, when the buck switch turns on, the current into pin 8 will suddenly increase to the
lower peak of the output current waveform, ramp up to the peak value, then drop to zero at turn-off. The average
input current during this on-time is the load current (150 mA). For a worst case calculation, C1 must supply this
average load current during the maximum on-time. To keep the input voltage ripple to less than 2V (for this
exercise), C1 calculates to:
(11)
Quality ceramic capacitors in this value have a low ESR which adds only a few millivolts to the ripple. It is the
capacitance which is dominant in this case. To allow for the capacitor’s tolerance, temperature effects, and
voltage effects, a 1.0 µF, 100V, X7R capacitor will be used.
C4: The recommended value is 0.022 µF for C4, as this is appropriate in the majority of applications. A high
quality ceramic capacitor, with low ESR is recommended as C4 supplies the surge current to charge the buck
switch gate at turn-on. A low ESR also ensures a quick recharge during each off-time. At minimum VIN, when
the on-time is at maximum, it is possible during start-up that C4 will not fully recharge during each 300 ns off-
time. The circuit will not be able to complete the start-up, and achieve output regulation. This can occur when the
frequency is intended to be low (e.g., R
ON
= 500K). In this case C4 should be increased so it can maintain
sufficient voltage across the buck switch driver during each on-time.
C5: This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at VIN. A low
ESR, 0.1µF ceramic chip capacitor is recommended, located close to the LM5009.
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