Datasheet

R
FB2
- 1
R
FB1
V
OUT
0.8V
=
1.25V
10 PA x t
RES
C
RES
=
0.8V
t
SS
x 10 PA
C
SS
=
Q
g
C
HB
t
'V
HB
¸
¹
·
¨
©
§
Z
IN
G
= x
Z
S
2
1
Z
S
R
IN
+ ESR
+
V
IN
2
Z
IN
=
P
OUT
LM25119/25119Q
SNVS680G –AUGUST 2010–REVISED JANUARY 2014 www.ti.com
(28)
The damping factor for the input filter is given by:
(29)
Where R
IN
is the input wiring resistance and ESR is the equivalent 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 negative, there is not enough resistance
in the circuit and the input filter will sustain an oscillation. When operating near the minimum 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 0.47 µF, and should be a good quality, low ESR,
ceramic capacitor located at the pins of the IC to minimize potentially damaging voltage transients caused by
trace inductance. A value of 1 μF was selected for this design.
BOOTSTRAP CAPACITOR
The bootstrap capacitor between the HB and SW pins supplies 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 capacitor 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 calculated as:
(30)
Q
g
is the high-side MOSFET gate charge and ΔV
HB
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 determined from:
(31)
For this application, a value of 0.047 μF was chosen for a soft-start time of 3.8 ms.
RESTART CAPACITOR
The restart pin sources 10 µA into the external restart capacitor (C
RES
). The value of the restart capacitor is given
by:
(32)
Where t
RES
is the time the LM25119 remains off before a restart attempt in hiccup mode current limiting. For this
application, a value of 0.47 µF was chosen for a restart time of 59 ms.
OUTPUT VOLTAGE DIVIDER
R
FB1
and R
FB2
set the output voltage level, the ratio of these resistors is calculated from:
(33)
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