Datasheet

MAX17094
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the current
ripple and therefore reduce the peak current, which
decreases core losses in the inductor and I
2
R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increase physical size and can increase I
2
R
losses in the inductor. Low inductance values decrease
the physical size but increase the current ripple and
peak current. Finding the best inductor involves choos-
ing the best compromise between circuit efficiency,
inductor size, and cost.
The equations used here include a constant called LIR,
which is the ratio of the inductor peak-to-peak ripple
current to the average DC inductor current at the full
load current. The best trade-off between inductor size
and circuit efficiency for step-up regulators generally
has an LIR between 0.3 and 0.5. However, depending
on the AC characteristics of the inductor core material
and ratio of inductor resistance to other power-path
resistances, the best LIR can shift up or down. If the
inductor resistance is relatively high, more ripple can
be accepted to reduce the number of turns required
and increase the wire diameter. If the inductor resis-
tance is relatively low, increasing inductance to lower
the peak current can decrease losses throughout the
power path. If extremely thin high-resistance inductors
are used, as is common for LCD panel applications, the
best LIR can increase to between 0.5 and 1.0.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions.
In Figure 2’s typical operating circuit, the LCD’s gate-on
and gate-off supply voltages are generated from two
unregulated charge pumps driven by the step-up regu-
lator’s LX node. The additional load on LX must there-
fore be considered in the inductance and current
calculations. The effective maximum output current,
I
MAIN(EFF)
, becomes the sum of the maximum load cur-
rent of the step-up regulator’s output plus the contribu-
tions from the positive and negative charge pumps:
where I
MAIN(MAX)
is the maximum step-up output cur-
rent, n
VN
is the number of negative charge-pump
stages, n
VP
is the number of positive charge-pump
stages, I
VN
is the negative charge-pump output cur-
rent, and I
VP
is the positive charge-pump output cur-
rent, assuming the initial pump source for I
VP
is V
MAIN
.
Calculate the approximate inductor value using the typ-
ical input voltage (V
IN
), the maximum output current
(I
MAIN(EFF)
), the expected efficiency (
η
TYP
) taken from
an appropriate curve in the Typical Operating
Characteristics , the desired switching frequency (f
OSC
),
and an estimate of LIR based on the above discussion:
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage V
IN(MIN)
using con-
servation of energy and the expected efficiency at that
operating point (η
MIN
) taken from an appropriate curve
in the Typical Operating Characteristics :
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX17094 LX current limit should exceed I
PEAK
and
the inductor’s DC current rating should exceed
I
IN(DC,MAX)
. For good efficiency, choose an inductor
with less than 0.1Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I
MAIN(MAX)
) is 300mA, with an 8V output
and a typical input voltage of 3.3V. The effective full-
load step-up current is:
Choose a switching frequency of 1.2MHz and a LIR of
0.36, and estimate the efficiency to be 85% at this operat-
ing point:
L
V
V
VV
AMHz
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
33
8
833
0 380 1 2
0
2
..
..
.- 885
036
41
.
.
⎛
⎝
⎜
⎞
⎠
⎟
≈ μH
ImAmAmAmA
MAIN EFF()
()=+×++×=300 1 20 2 1 20 380
II
I
PEAK IN DC MAX
RIPPLE
=+
(, )
2
I
VVV
LV f
RIPPLE
IN MIN MAIN IN MIN
MAIN O
=
×
()
××
() ()
-
SSC
I
IV
V
IN DC MAX
MAIN EFF MAIN
IN MIN MIN
(, )
()
()
=
×
×η
L
V
V
VV
If
IN
MAIN
MAIN IN
MAIN EFF OSC
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
2
-
()
⎜⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
η
TYP
LIR
II nInI
MAIN EFF MAIN MAX VN VN VP VP() ( )
()=+×++×1
Internal-Switch Boost Regulator with Integrated
7-Channel Driver, VCOM Calibrator, Op Amp, and LDO
20 ______________________________________________________________________________________