C1 =

'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.

calculated from:

(8)

calculated from:

(9)

calculated above.

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:

(10)

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

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.

12

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