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L4992

TRIPLE OUTPUT POWER SUPPLY CONTROLLER

STMicroelectronics 

L4992

DESIGN PROCEDURE (continued)

which affects efficiency at low load currents. Besides, this current is drawn from the REG5 line whose

source capability, ISRC (25mA min), must not be exceeded, thus a further constraint concernes the MOS-

FET total gate charge (@ Vgs = 5V);

assuming four identical MOSFET’s.

Qg

≤

ISRC

4 ⋅ fSW

,

The Schottky diode to be placed in parallel to the synchronous rectifier must have a reverse voltage

VRRM greater than Vinmax. Since it conducts for less than 5% of the switching period, the current rating

can be much lower than Iout. The selection criterion should be:

Sense Resistors

Vf(schottky) < Vf(body-diode) @ I = ILpk

The sense resistor of each section is selected according to their respective maximum output current. The

current sense comparator limits the inductor peak current and therefore the maximum DC output current

is the peak value less half of the peak-to-peak ripple. The intervention threshold is set at 100 mV for both

sections, thus the resistor values should be:

Rsense5

=

100

IL5pk

[mΩ]

Rsense3

=

100

IL3pk

[mΩ]

Since the comparator threshold that triggers pulse-skipping mode is 26mV, the output current at which

the system enters this kind of operation is approximately one fourth of the maximum output current.

The sense resistors values are in the low milliohms thus it is important to take correctly the current sense

signals. Make sure that the Kelvin connections between the current sense pins of the IC and the sense

resistor do not carry the output current.

Input Capacitors

A pulsed current (with zero average value) flows through the input capacitor of a buck converter. The AC

component of this current is quite high and dissipates a considerable amount of power on the ESR of the

capacitor:

PCin

=

ESR

⋅

Io2ut

⋅

Vout

⋅

(Vin −

Vi2n

Vout)

It is easy to find that PCin has a maximum equal to (1/2) ⋅ Iout (@ Vin=2 ⋅ Vout, that is, 50% duty cycle).

The input capacitor of each section, therefore, should be selected for a RMS ripple current rating as high

as half the respective maximum output current.

The capacitance value is not very important but in reality a minimum value must be ensured for stability

reasons. In fact, switching regulators exhibit a negative input impedance that, at low frequencies, is:

Zin(DC)

=

−

Vi2n

Vout ⋅ Iout

thus, if the impedance of the power source is not well below the absolute value of Zin(DC) at frequencies

up to the bandwidth of the regulator control loop, there is the possibility for oscillations. To ensure stabil-

ity, the following condition must be satisfied:

Cin

>>⋅ESRin

Leq

⋅ | Zin(DC)

|

where Leq is the inductance of the circuit upstream the switching regulator input and ESRin is related to

the input capacitor itself.

The use of high performance electrolytic capacitors is recommended. If a higher cost is of no concern,

OS-CON capacitors are an excellent choice because they offer the smallest size for a given ESR or cur-

rent rating. Tantalum capacitors do not tolerate pulsed current, so their use is not advisable.

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