Rainbow Electronics MAX17061 User Manual
Page 22
MAX17061
The peak-inductor current at minimum input voltage is
calculated as follows:
Alternatively, choosing a DCM operating mode at 750kHz
and estimating efficiency of 85% at this operating point:
A 4.7µH inductor is chosen. The peak inductor current
at minimum input voltage is calculated as follows:
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
on the output capacitor, and the ohmic ripple due to the
capacitor’s equivalent series resistance (ESR):
and:
where I
PEAK
is the peak inductor current (see the
Inductor Selection
section).
The output voltage ripple should be low enough for the
FB_ current-source regulation. The ripple voltage should
be less than 200mV
P-P
. For ceramic capacitors, the out-
put voltage ripple is typically dominated by V
RIPPLE(C)
.
The voltage rating and temperature characteristics of the
output capacitor must also be considered. The actual
capacitance of a ceramic capacitor is reduced by DC
voltage biasing. Ensure the selected capacitor has
enough capacitance at actual DC biasing.
Rectifier Diode Selection
The MAX17061’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. The diode should be
rated to handle the output voltage and the peak switch
current. Make sure that the diode’s peak current rating
is at least I
PEAK
calculated in the
Inductor Selection
section and that its breakdown voltage exceeds the
output voltage.
Overvoltage Protection Determination
The OV protection circuit should ensure the circuit safe
operation; therefore, the controller should limit the out-
put voltage within the ratings of all MOSFET, diode, and
output capacitor components, while providing sufficient
output voltage for LED current regulation. The OV pin is
tied to the center tap of a resistive voltage-divider (R1
and R2 in Figure 1) from the high-voltage output. When
the controller detects the OV pin voltage reaching the
threshold V
OV_TH
, typically 1.23V, OV protection is acti-
vated. Hence, the step-up converter output overvoltage
protection point is:
In Figure 1, the output OVP voltage is set to:
Input Capacitor Selection
The input capacitor (C
IN
) filters the current peaks
drawn from the input supply and reduces noise injec-
tion into the IC. A 10µF ceramic capacitor is used in the
Typical Operating Circuit
(Figure 1) because of the
high source impedance seen in typical lab setups.
Actual applications usually have much lower source
impedance since the step-up regulator often runs
directly from the output of another regulated supply. In
some applications, C
IN
can be reduced below the val-
ues used in the
Typical Operating Circuit
(Figure 1).
Ensure a low-noise supply at IN by using adequate C
IN
.
Alternatively, greater voltage variation can be tolerated
on C
IN
if IN is decoupled from C
IN
using an RC low-
pass filter.
V
V
M
k
V
OUT OVP
(
)
.
(
.
.
)
=
× +
≈
1 236
1
2 21
61 9
45
Ω
Ω
V
V
R
R
OUT OVP
OV TH
(
)
_
=
× +
⎛
⎝⎜
⎞
⎠⎟
1
1
2
V
I
R
RIPPLE ESR
PEAK ESR COUT
(
)
(
)
≈
V
I
C
V
V
V
f
RIPPLE C
OUT MAX
OUT
OUT MAX
IN MIN
OUT MAX OSC
( )
(
)
(
)
(
)
(
)
≈
−
⎛
⎝
⎜
⎞
⎠
⎟
V
V
V
RIPPLE
RIPPLE C
RIPPLE ESR
=
+
( )
(
)
I
mA
V
V
V
V
uH
MHz
V
V
A
PEAK
=
Ч Ч
Ч
+
−
(
)
Ч
Ч
Ч
+
(
)
=
100
2 35 9
35 9
0 4
7
4 7
0 675
0 85
35 9
0 4
1 47
.
.
.
.
.
.
.
.
.
L
V
V
V
V
MHz
V
mA
H
DCM MAX
(
)
.
.
(
)
.
.
.
.
= −
+
⎛
⎝⎜
⎞
⎠⎟
Ч
Ч
Ч
Ч
Ч
=
1
7
35 9
0 4
7
0 85
2 0 825
35 9
100
5 6
2
μ
I
mA
V
V
V
V
V
H
V
MHz
A
PEAK
=
Ч
Ч
+
Ч
−
(
)
Ч
Ч
Ч
=
100
35 9
7
0 85
7
35 9
7
2 10
35 9
0 9
0 92
.
.
.
.
.
.
μ
8-String White LED Driver with
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22
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