MAX1513 Datasheet, PDF (18/28 Pages) Maxim Integrated Products – TFT-LCD Power-Supply Controllers

TFT-LCD Power-Supply Controllers
taken from an appropriate curve in the Typical
Operating Characteristics, and an estimate of LIR
based on the above paragraphs:
L
=
⎛
⎝⎜
V_IN
V_MAIN
⎞
⎠⎟
2_⎛
⎜
⎝
V_MAIN
I_MAIN(EFF)
- V_IN
× f_OSC
⎞
⎟
⎠
⎛
⎝⎜
η_TYP
LIR
⎞
⎠⎟
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:
I_{IN(DC, MAX)}
=
I_MAIN(EFF) × V_MAIN
V_IN(MIN) × η_MIN
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:
( ) _IRIPPLE
=
V_IN(MIN) × V_MAIN - V_IN(MIN)
L × V_MAIN × f_OSC
I_PEAK
=
I_{IN(DC, MAX)} +
I_RIPPLE
2
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:
L
=
⎛ 5V ⎞ ²âŽ› 15V - 5V ⎞ ⎛ 0.85⎞
⎝⎜ 15V ⎠⎟ ⎝⎜ 0.5A × 1.5MHz ⎠⎟ ⎝⎜ 0.6 ⎠⎟
≈
2.2µH
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 80% at that operating point:
I_{IN(DC, MAX)}
=
0.5A × 15V
4.5V × 0.8
≈
2.1A
The ripple current and the peak current are:
4.5V × ₍15V - 4.5V₎
I_RIPPLE = _{2.2µH × 15V × 1.5MHz} ≈ 1.0A
I_PEAK
= 2.1A +
1.0A
2
≈ 2.6A
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:
τ= ^L
R_L(TYP)
2) Determine the component values of the sense net-
work. Select C_S, and then calculate R_S using:
R_S
=
τ
C_S
3) Calculate the worst-case high sense voltage over
temperature using the maximum DCR value (R_L(MAX))
found in the inductor technical specifications:
( ) ^V_SENSE ^{= I}_PEAK ^{× R}_L(MAX) ^{× 1 + TC × ∆T}
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.
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