Rainbow Electronics MAX17117 User Manual
Page 19
Internal-Switch Boost Regulator with Integrated
7-Channel Scan Driver, Op Amp, and LDO
MAX17117
______________________________________________________________________________________ 19
Choosing an LIR of 0.2 and estimating efficiency of 85%
at this operating point:
2
3.3V
8.5V 3.3V
0.85
L
9.7 H
8.5V
0.285A 1.2MHz
0.2
−
=
≈
µ
×
A 10FH inductor is chosen. Then, using the circuit’s
minimum input voltage (3.0V) and estimating efficiency
of 83% at that operating point:
IN(DC,MAX)
0.285A 8.5V
I
0.973A
3V 0.83
Ч
=
≈
Ч
The ripple current and the peak current at that input volt-
age are:
(
)
RIPPLE
3V
8.5V 3V
I
0.162A
10 H 8.5V 1.2MHz
×
−
=
≈
µ Ч
Ч
PEAK
0.162A
I
0.973A
1.05A
2
=
+
=
Peak Inductor Current-Limit Setting
Connecting R
ENA
between the ENA pin and the LDOO
output, as shown in Figure 1, allows the inductor peak
current limit to be adjusted up to 2A max by choosing the
appropriate R
ENA
resistor with the following equation:
LDOO
ENA
OCP
(V
1.25V)(80000)
R
I
−
≈
The above threshold set by R
ENA
varies depending on
the step-up converter’s input voltage, output voltage,
and duty cycle. Place R
ENA
close to the IC such that the
connection between R
ENA
and the ENA pin is as short
as possible.
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):
RIPPLE
RIPPLE(C)
RIPPLE(ESR)
V
V
V
=
+
MAIN
MAIN
IN
RIPPLE(C)
OUT
MAIN OSC
I
V
V
V
C
V
f
−
≈
and:
RIPPLE(ESR)
PEAK ESR(COUT)
V
I
R
≈
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
RIPPLE(C)
. The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
Input-Capacitor Selection
The input capacitor (C3) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion into the IC. A 10FF ceramic capacitor is used in the
typical application 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.
Rectifier Diode
The MAX17117 high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. In general, a 1A Schottky diode
complements the internal MOSFET well.
Output-Voltage Selection
The output voltage of the main step-up regulator is
adjusted by connecting a resistive voltage-divider from
the output (V
MAIN
) to AGND with the center tap con-
nected to FB (see Figure 1). Select R2 in the 10kI to
50kI range. Calculate R1 with the following equation:
MAIN
REF
V
R1 R2
1
V
=
×
−
Place R1
and R2 close to the IC such that the connec-
tions between these components and the FB pin are kept
as short as possible.
Loop Compensation
Choose R
COMP
to set the high-frequency integrator gain
for fast-transient response. Choose C
COMP
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:
IN
MAIN
OUT
COMP
MAIN(MAX)
1.45k V
V
C
R
L I
Ч
Ч
Ч
≈
Ч
MAIN
MAIN(MAX)
COMP
2
IN
COMP
40 V
L I
C
(V )
R
Ч
Ч Ч
≈
Ч
To further optimize transient response, vary R
COMP
in
20% steps and C
COMP
in 50% steps while observing
transient-response waveforms.