lm25119psqx National Semiconductor Corporation, lm25119psqx Datasheet - Page 17

• Manufacturer Part Number: lm25119psqx
• Description: Lm25119 Wide Input Range Dual Synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

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
Seven 2.2μF 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:
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
characteristic source impedance (Z
(f
Where L
exhibits negative input impedance which is lowest at the min-
imum input voltage:
The damping factor for the input filter is given by:
Where R
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
bench test setup.
S
) are:
IN
IN
IN
is the input wiring resistance and ESR is the equiv-
is the inductance of the input wire. The converter
may be needed to damp the input for a typical
S
) and resonant frequency
IN
> 10 x L
RMS
> I
OUT
IN
. The
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VCC CAPACITOR
The primary purpose of the VCC capacitor (C
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
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 1μF 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
0.1μF, 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:
Q
tolerable voltage droop on C
5% of VCC. A value of 0.47μF was selected for this design.
SOFT START CAPACITOR
The capacitor at the SS pin (C
time (t
final regulated value. The value of C
termined from:
For this application, a value of 0.047μF was chosen for a soft-
start time of 3.8ms.
RESTART CAPACITOR
The restart pin sources 10µA into the external restart capac-
itor (C
Where t
restart attempt in hiccup mode current limiting. For this appli-
cation, a value of 0.47µF was chosen for a restart time of
59ms.
OUTPUT VOLTAGE DIVIDER
R
resistors is calculated from:
2.21kΩ was chosen for R
R
is to select the value of R
10kΩ. The value of R
total divider power dissipation small.
FB1
FB2
g
is the high-side MOSFET gate charge and ΔV
and R
value of 6.98kΩ for V
SS
RES
), which is the time for the output voltage to reach the
RES
). The value of the restart capacitor is given by:
VCC
FB2
is the time the LM25119 remains off before a
should be no smaller than 0.47µF, and should
set the output voltage level, the ratio of these
FB1
should be large enough to keep the
FB1
FB1
OUT1
in the range between 500Ω and
in this design which results in a
HB
, which is typically less than
SS
of 3.3V. A reasonable guide
) determines the soft-start
SS
for a given time is de-
VCC
) is to supply
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HB
is the
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