LM5119 datasheet(17/24 Pages) NSC

17 / 24 page

INPUT CAPACITORS

The regulator input supply voltage typically has high source

impedance at the switching frequency. Good quality input ca-

pacitors are necessary to limit the ripple voltage at the VIN

pin while supplying most of the switch current during the on-

time. When the buck switch turns on, the current into the buck

switch steps to the valley of the inductor current waveform,

ramps up to the peak value, and then drops to the zero at turn-

off. The input capacitance should be selected for RMS current

rating and minimum ripple voltage. A good approximation for

the required ripple current rating necessary is I

RMS > IOUT / 2.

Seven 2.2

μF ceramic capacitors were used for each channel.

With ceramic capacitors, the input ripple voltage will be trian-

gular. The input ripple voltage with one channel operating is

approximately:

(24)

(25)

The ripple voltage of the input capacitors will be reduced sig-

nificantly with dual channel operation since each channel

operates 180 degrees out of phase from the other. Capacitors

connected in parallel should be evaluated for RMS current

rating. The current will split between the input capacitors

based on the relative impedance of the capacitors at the

switching frequency.

When the converter is connected to an input power source, a

resonant circuit is formed by the line inductance and the input

capacitors. To minimize overshoot make C

IN > 10 x LIN. The

characteristic source impedance (Z

S) and resonant frequency

S) are:

(26)

(27)

Where L

IN is the inductance of the input wire. The converter

exhibits negative input impedance which is lowest at the min-

imum input voltage:

(28)

The damping factor for the input filter is given by:

(29)

Where R

IN is the input wiring resistance and ESR is the equiv-

alent series resistance of the input capacitors. When

δ = 1,

the input filter is critically damped. This may be difficult to

achieve with practical component values. With

δ < 0.2, the

input filter will exhibit significant ringing. If

δ is zero or nega-

tive, there is not enough resistance in the circuit and the input

filter will sustain an oscillation. When operating near the min-

imum input voltage, a bulk aluminum electrolytic capacitor

across C

IN may be needed to damp the input for a typical

bench test setup.

VCC CAPACITOR

The primary purpose of the VCC capacitor (C

VCC) is to supply

the peak transient currents of the LO driver and bootstrap

diode as well as provide stability for the VCC regulator. These

peak currents can be several amperes. The recommended

value of C

VCC should be no smaller than 0.47µF, and should

be a good quality, low ESR, ceramic capacitor located at the

pins of the IC to minimize potentially damaging voltage tran-

sients caused by trace inductance. A value of 1

μF was se-

lected for this design.

BOOTSTRAP CAPACITOR

The bootstrap capacitor between the HB and SW pins sup-

plies the gate current to charge the high-side MOSFET gate

at each cycle’s turn-on and recovery charge for the bootstrap

diode. These current peaks can be several amperes. The

recommended value of the bootstrap capacitor is at least

0.1

μF, and should be a good quality, low ESR, ceramic ca-

pacitor located at the pins of the IC to minimize potentially

damaging voltage transients caused by trace inductance. The

absolute minimum value for the bootstrap capacitor is calcu-

lated as:

(30)

g is the high-side MOSFET gate charge and ΔVHB is the

tolerable voltage droop on C

HB, which is typically less than

5% of VCC. A value of 0.47

μF was selected for this design.

SOFT START CAPACITOR

The capacitor at the SS pin (C

SS) determines the soft-start

time (t

SS), which is the time for the output voltage to reach the

final regulated value. The value of C

SS for a given time is de-

termined from:

(31)

For this application, a value of 0.047

μF was chosen for a soft-

start time of 3.8ms.

RESTART CAPACITOR

The restart pin sources 10µA into the external restart capac-

itor (C

RES). The value of the restart capacitor is given by:

(32)

Where t

RES is the time the LM5119 remains off before a restart

attempt in hiccup mode current limiting. For this application,

a value of 0.47µF was chosen for a restart time of 59ms.

OUTPUT VOLTAGE DIVIDER

FB1 and RFB2 set the output voltage level, the ratio of these

resistors is calculated from:

(33)

1.33k

Ω was chosen for R

FB1 in this design which results in a

FB2 value of 6.98kΩ for VOUT2 of 5V. A reasonable guide is

to select the value of R

FB1 in the range between 500Ω and

10k

Ω. The value of R

FB1 should be large enough to keep the

total divider power dissipation small.

LM5119 Datasheet(PDF) 17 Page - National Semiconductor (TI)

LM5119 Datasheet(HTML) 17 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL