Datasheet

A 4.1μH inductor is chosen. Then, using the circuit’s
minimum input voltage (3.0V) and estimating efficiency
of 82% at that operating point:
The ripple current and the peak current at that input
voltage are:
Setting the Switching Frequency
To set the switching frequency, connect a resistor from
FREQ to AGND. Calculate the resistor value in kΩ from
the following equation:
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
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
RIPPLE(C)
.
The voltage rating and temperature characteristics of
the output capacitor must also be considered.
Input Capacitor Selection
The input capacitor (C4) reduces 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 2) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source imped-
ance since the step-up regulator often runs directly
from the output of another regulated supply. Typically,
C4 can be reduced below the values used in the typical
operating circuit. Ensure a low noise supply at IN by
using an adequate value for C4.
Rectifier Diode
The MAX17094’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 3A 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 2). Select R2 in the 10kΩ to
50kΩ range. Calculate R1 with the following equation:
where V
REF
, the step-up regulator’s feedback set point,
is 1.235V (typical). Place R1 and R2 close to the IC.
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:
To further optimize transient response, vary R
COMP
in
20% steps and C
COMP
in 50% steps while observing
transient-response waveforms.
Setting the LDO Output Voltage
The output voltage of the LDO is adjusted by connect-
ing a resistive voltage-divider from the output (V
LOUT
)
to AGND with the center tap connected to FBL (see
Figure 2). Select R8 in the 10kΩ to 50kΩ range.
Calculate R7 with the following equation:
Place R7 and R8 close to the IC.
Connect to a 1μF capacitor between LIN and AGND to
keep the source impedance to the LDO low and con-
nect a 4.7μF low equivalent-series-resistance (ESR)
capacitor between LOUT and AGND to ensure stability
and to provide good output-transient performance.
RR
V
V
LOUT
78
0 618
1=×
⎛
⎝
⎜
⎞
⎠
⎟
.
-
C
VLI
VR
COMP
MAIN MAIN MAX
IN COMP
≈
×××
×
10
2
()
()
R
VV C
LI
COMP
IN MAIN OUT
MAIN MAX
≈
×× ×
×
250
()
RR
V
V
MAIN
12
1 235
1=×
⎛
⎝
⎜
⎞
⎠
⎟
.
-
VIR
RIPPLE ESR PEAK ESR COUT() ( )
≈
V
I
C
VV
Vf
RIPPLE C
MAIN
OUT
MAIN IN
MAIN OSC
()
≈
⎛
⎝
⎜
⎞
⎠
-
⎟⎟
VV V
RIPPLE RIPPLE C RIPPLE ESR
=+
() ( )
fR
MHZ FREQ k() ()
.=×0 015
Ω
I
VVV
μH V MHz
A
I
RIPPLE
PE
=
×
()
××
≈
383
41 8 12
0 381
-
..
.
AAK
A
A
A=+ =124
0 381
2
143.
.
.
I
AV
V
A
IN DC MAX(, )
.
.
.=
×
×
≈
038 8
3082
124
MAX17094
Internal-Switch Boost Regulator with Integrated
7-Channel Driver, VCOM Calibrator, Op Amp, and LDO
______________________________________________________________________________________ 21