Datasheet

üV
OUT
= 13.3mV
1
2
¸
¹
·
¨
©
§
8 x 230 kHz x 470 PF
'
V
OUT
= 1.32A x 0.01:
2
+
1
2
¸
¹
·
¨
©
§
8 x x C
OUT
SW
´
¶
'V
OUT
= I
PP
x ESR
2
+
15 PH
10 x 0.01: x 2.5 x 820 pF
R
RAMP
=
= 73.2 k:
L
10 x R
S
x K x C
RAMP
R
RAMP
=
0.01:
15 PH
55V x 100 ns
+
I
LIM_PEAK
=
0.12
= 12.37A
LM5119/LM5119Q
www.ti.com SNVS676F –AUGUST 2010–REVISED FEBRUARY 2013
(18)
Where t
ON(MIN)
is the minimum HO on-time which is nominally 100ns. The chosen inductor must be evaluated for
this condition, especially at elevated temperature where the saturation current rating of the inductor may drop
significantly. At the maximum input voltage with a shorted output, the valley current must fall below V
CS(TH)
/ R
S
before the high-side MOSFET is allowed to turn on.
RAMP RESISTOR AND RAMP CAPACITOR
The value of ramp capacitor (C
RAMP
) should be less than 2nF to allow full discharge between cycles by the
discharge switch internal to the LM5119. A good quality, thermally stable ceramic capacitor with 5% or less
tolerance is recommended. For this design the value of C
RAMP
was set at the standard capacitor value of 820pF.
With the inductor, sense resistor and the K factor selected, the value of the ramp resistor (R
RAMP
) can be
calculated as:
(19)
(20)
The standard value of 73.2kΩ was selected.
OUTPUT CAPACITORS
The output capacitors smooth the inductor ripple current and provide a source of charge during transient loading
conditions. For this design example, a 470µF electrolytic capacitor with 10mΩ ESR was selected as the main
output capacitor. The fundamental component of the output ripple voltage is approximated as:
(21)
(22)
(23)
Two 22µF low ERS / ESL ceramic capacitors are placed in parallel with the 470µF electrolytic capacitor, to
further reduce the output voltage ripple and spikes.
Table 1. Performance Variation by K Factor
K < 1 1 <— K —> 3 K > 3
Cross Talk Higher Lower
Peak Inductor Current with Short Output Lower Higher
Introduces additional
Sub-harmonic
Condition
pole near cross-over
oscillation may occur
frequency
Inductor Size Smaller Larger
Power Dissipation of Rs Higher Lower
Efficiency Lower Higher
INPUT CAPACITORS
The regulator input supply voltage typically has high source impedance at the switching frequency. Good quality
input capacitors 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
> I
OUT
/ 2. Seven 2.2μF ceramic capacitors were used for each
channel. With ceramic capacitors, the input ripple voltage will be triangular. The input ripple voltage with one
channel operating is approximately:
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