Rainbow Electronics MAX17075 User Manual
Page 19
The inductor’s saturation current rating and the
MAX17075’s LX current limit should exceed I
AVDD
_
PEAK
,
and the inductor’s DC current rating should exceed
I
IN(DC,MAX)
. For good efficiency, choose an inductor with
less than 0.1
Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I
AVDD(MAX)
) is 500mA with a 13V output
and a typical input voltage of 5V. Choosing an LIR of 0.5
and estimating efficiency of 85% at this operating point:
Using the circuit’s minimum input voltage (2.5V) and
estimating efficiency of 80% at that operating point:
The ripple current and the peak current are:
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, 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). For ceramic capacitors, the
output voltage ripple is typically dominated by
V
AVDD
_
RIPPLE(C)
. The voltage rating and temperature
characteristics of the output capacitor must also be
considered.
Input-Capacitor Selection
The input capacitor (C
IN
) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion into the IC. Two 10µF ceramic capacitors are 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.
Typically, C
IN
can be reduced below the values used in
the typical operating circuit. Ensure a low-noise supply
at V
CC
by using adequate C
IN
. Alternately, greater volt-
age variation can be tolerated on C
IN
if VCC is decou-
pled from C
IN
using an RC lowpass filter (see R1 and
C5 in Figure 1).
Rectifier Diode
The MAX17075’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. In general, a 2A Schottky
diode complements the internal MOSFET well.
Output Voltage Selection
The output voltage of the step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (V
AVDD
) to ground with the center tap con-
nected to FB (see Figure 1). Select R9 in the 10k
Ω to
50k
Ω range. Calculate R8 with the following equation:
where V
FB
, the step-up regulator’s feedback set point,
is 1.25V. Place R8 and R9 close to the IC.
Loop Compensation
Choose R
COMP
(R10 in Figure 1) to set the high-fre-
quency integrator gain for fast-transient response.
Choose C
COMP
(C12 in Figure 1) to set the integrator
zero to maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
C
V
C
I
R
COMP
AVDD
AVDD
AVDD MAX
COMP
≈
Ч
Ч
10
(
)
R
V
V
C
L
I
COMP
IN
AVDD
AVDD
AVDD
AVDD MAX
≈
Ч
Ч
Ч
Ч
312 5
.
(
)
R
R
V
V
AVDD
FB
8
9
1
=
×
−
⎛
⎝⎜
⎞
⎠⎟
V
I
R
AVDD RIPPLE ESR
PEAK ESR AVDD
_
(
)
_
≈
V
I
C
V
V
V
f
AVDD RIPPLE C
AVDD
AVDD
AVDD
IN
AVDD SW
_
( )
≈
−
⎛⎛
⎝⎜
⎞
⎠⎟
,
V
V
V
AVDD RIPPLE
AVDD RIPPLE C
AVDD RIPPLE E
_
_
( )
_
(
=
+
S
SR)
I
A
A
A
PEAK
=
+
≈
3 25
0 51
2
3 51
.
.
.
I
V
V
V
µH
V
MHz
RIPPLE
=
Ч
−
(
)
Ч
Ч
≈
2 5
13
2 5
3 3
13
1 2
0 5
.
.
.
.
. 1
1A
I
A
V
V
A
IN DC MAX
(
,
)
.
.
.
.
=
Ч
Ч
≈
0 5
13
2 5
0 8
3 25
L
V
V
V
V
A
MHz
AVDD
= ⎛
⎝⎜
⎞
⎠⎟
−
×
⎛
⎝⎜
⎞
⎠⎟
5
13
13
5
0 5
1 2
0
2
.
.
.8
85
0 5
3 35
.
.
⎛
⎝⎜
⎞
⎠⎟
≈
μH
MAX17075
Boost Regulator with Integrated Charge Pumps,
Switch Control, and High-Current Op Amp
______________________________________________________________________________________
19