Datasheet
I
OUT(MAX)
C
IN
8 x g
SW
x ûV
IN
>
OUT
C
L(MIN) REF
V
25mV
R
I V
t u
'
ûI
L
C
OUT
=
8 x g
sw
x ûV
ripple
+
+
+
V
IN
BST
RON
RTN
SW
VCC
FB
VIN
VOUT
R
FB1
R
C
R
UV1
R
ON
C
OUT
C
BS
T
C
IN
R
FB2
R
UV2
L1
UVLO
+
C
VCC
LM5019
7.5V-100V
1
2
3
4
5
6
8
7
Shutdown
+
C
BYP
LM5019
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SNVS788E –JANUARY 2012–REVISED DECEMBER 2013
Figure 15. Reference Schematic for Selection of External Components
Output Capacitor:
The output capacitor is selected to minimize the capacitive ripple across it. The maximum ripple is observed at
maximum input voltage and is given by:
where
• ΔV
ripple
is the voltage ripple across the capacitor (11)
Substituting ΔV
ripple
= 10mV gives C
OUT
= 2.94 μF. A 4.7 μF standard value is selected. An X5R or X7R type
capacitor with a voltage rating 16 V or higher should be selected.
Series Ripple Resistor R
C
:
The series resistor should be selected to produce sufficient ripple at the feedback node. The ripple produced by
R
C
is proportional to the inductor current ripple, and therefore R
C
should be chosen for minimum inductor current
ripple which occurs at minimum input voltage. The R
C
is calculated by the equation:
(12)
This gives an R
C
of greater than or equal to 10.8 Ω. Selecting R
C
= 11 Ω results in ~1 V of maximum output
voltage ripple. For applications requiring lower output voltage ripple, Type II or Type III ripple injection circuits
should be used as described in the section Ripple Configuration.
V
CC
and Bootstrap Capacitor:
The V
CC
capacitor provides charge to bootstrap capacitor as well as internal circuitry and low side gate driver.
The Bootstrap capacitor provides charge to high side gate driver. A good value for C
VCC
is 1 μF. A good value for
C
BST
is 0.01 μF.
Input Capacitor:
Input capacitor should be large enough to limit the input voltage ripple:
(13)
choosing a ΔV
IN
= 0.5 V gives a minimum C
IN
= 0.12 μF. A standard value of 1 μF is selected. The input
capacitor should be rated for the maximum input voltage under all conditions. A 100 V, X7R dielectric should be
selected for this design.
Input capacitor should be placed directly across V
IN
and RTN (pin 2 and 1) of the IC. If it is not possible to place
all of the input capacitor close to the IC, a 0.1 μF capacitor should be placed near the IC to provide a bypass
path for the high frequency component of the switching current. This helps limit the switching noise.
UVLO Resistors:
The UVLO resistors R
FB1
and R
FB2
set the UVLO threshold and hysteresis according to the following relationship:
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