Datasheet

MAX1513/MAX1514
TFT-LCD Power-Supply Controllers
18 ______________________________________________________________________________________
taken from an appropriate curve in the Typical
Operating Characteristics, and an estimate of LIR
based on the above paragraphs:
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 the
following equation:
The expected efficiency at that operating point (η
MIN
)
can be 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
MAX1513/MAX1514s’ current limit (I
LIM
) should exceed
I
PEAK
, and the inductor’s DC current rating should
exceed I
IN(DC, MAX)
.
Considering the typical operating circuit, the maximum
load current (I
MAIN(MAX)
) is 400mA for I
MAIN
directly
and 30mA for REG G to provide V
GAMMA
. The one-
stage negative charge pump provides 30mA to REG N
for V
GOFF
, and the one-stage positive charge pump
provides 20mA to REG P for V
GON
. Altogether, the
effective maximum output current (I
MAIN(EFF)
) is 500mA
with a 15V output and a typical 5V input voltage. The
switching frequency is set to 1.5MHz. Choosing an
LIR of 0.6 and estimating efficiency of 85% at this
operating point:
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 80% at that operating point:
The ripple current and the peak current are:
The inductor DCR should be low enough for reasonable
efficiency. As a rule of thumb, do not allow the voltage
drop across the inductor DCR to exceed a few percent
of the input voltage at I
PEAK
.
Many notebook panel designs have height constraints
on the components. If a thin inductor with the required
current rating is not available, use two thin inductors in
series or parallel.
Current-Sense Network Selection
After selecting the inductor, use the following steps to
design the current-sense network for lossless current
sensing.
1) Calculate the RC time constant of the sense network
using the typical inductance and typical DCR:
2) Determine the component values of the sense net-
work. Select C
S
, and then calculate R
S
using:
3) Calculate the worst-case high sense voltage over
temperature using the maximum DCR value (R
L(MAX)
)
found in the inductor technical specifications:
where I
PEAK
is the peak inductor current calculated in
the Inductor Selection section, TC is the temperature
coefficient of copper (0.5%/°C) and ∆T is the difference
between the specified temperature for R
L(MAX)
and the
maximum expected inductor temperature.
4) Compare the calculated sense voltage with the mini-
mum value of the current-limit threshold in the Electrical
Characteristics (100mV). If the sense voltage is between
80mV and 100mV, use the current-sense configuration
in Figure 8 with the calculated C
S
and R
S
above.
VIR TCT
SENSE PEAK L MAX ()
=× ×+×
()
1 ∆
R
C
S
S
=
τ
τ
()
=
L
R
L TYP
I
VVV
H V MHz
A
IA
A
A
RIPPLE
PEAK
. .
. .
.
.
.
.
=
×
()
××
≈
=+ ≈
45 15 45
22 15 15
10
21
10
2
26
-
µ
I
AV
V
A
IN DC MAX(, )
.
. .
.=
×
×
≈
05 15
45 08
21
L
V
V
VV
A MHz
H
. .
.
.
.=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
≈
5
15
15 5
05 15
085
06
22
2
-
µ
I
VVV
LV f
II
I
RIPPLE
IN MIN MAIN IN MIN
MAIN OSC
PEAK IN DC MAX
RIPPLE
() ()
(, )
=
×
()
××
=+
-
2
I
IV
V
IN DC MAX
MAIN EFF MAIN
IN MIN MIN
(, )
()
()
=
×
×η
L
V
V
VV
I f LIR
IN
MAIN
MAIN IN
MAIN EFF OSC
TYP
()
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
2
- η