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MAX1998 Datasheet

  • MAX1998

  • Quintuple/Triple-Output TFT LCD Power Supplies with Fault Pr...

  • Maxim   Maxim

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Quintuple/Triple-Output TFT LCD Power Supplies
with Fault Protection and VCOM Buffer
MAX1997/MAX1998
ONDC
SHDN
UVLO
REFERENCE READY
THERMAL FAULT
R
Q
S
FAULT
LATCH
GATE
READY
ENABLES REG 1 LINEAR-REGULATOR
GATE
OVERCURRENT FAULT
REG 2 FAULT
REG 1 FAULT
REG P FAULT
STEP-UP REGULATOR FAULT
REG N FAULT
f
OSC
/128
FREQ
PFLT
V
CT
> V
ONP
CLK
R
RIPPLE
COUNTER
STEP-UP REGULATOR
SOFT-START DONE
V
CT
> V
ONN
V
CT
> V
ON2
ENABLES REG 2 LINEAR-REGULATOR
ENABLES STEP-UP REGULATOR
ENABLES REG P LINEAR-REGULATOR
ENABLES REG N LINEAR-REGULATOR
FAULT TIMER
Figure 8. Startup and Fault Protection Logic
The maximum inductor current, input voltage, output
voltage, and switching frequency determine the induc-
tor value. To ensure an adequate inductor current-
sense signal in the IC, always calculate the inductor
value with the maximum guaranteed inductor current
even though the actual operating current may be much
lower. For the MAX1997/MAX1998, the maximum guar-
anteed inductor current is the minimum value of the
internal LX current limit (1.6A, see the
Electrical
Characteristics).
The equations provided here include a
constant defined as LIR, which is the ratio of the peak-
to-peak inductor current ripple to the average DC
inductor current. For a good compromise between the
size of the inductor, power loss, and output voltage rip-
ple, select an LIR of 0.3 to 0.5. The inductance value is
then given by:
錚?/div>
V
IN(TYP)
錚?錚?/div>
V
MAIN
- V
IN(TYP)
錚?錚?/div>
1
錚?/div>
L
=錚?/div>
錚?/div>
錚鳳，
錚鳳，
錚?/div>
V
MAIN
錚?錚?/div>
I
L(MAX)
f
OSC
錚?錚?/div>
LIR
錚?/div>
where f
OSC
is the oscillator frequency (see
Elec-
trical Characteristics),
and I
L(MAX)
is 1.6A. Considering
the typical application circuit, the typical input voltage
is 3.3V, the main output voltage is 9V, and the switching
frequency is 1.5MHz. Based on the above equations,
the inductance value is 4.3碌H for an LIR of 0.2. The
inductance value is 1.7碌H for an LIR of 0.5. The induc-
tance in the standard application circuit is chosen to be
3.3碌H.
The inductor鈥檚 peak current rating should be higher
than the expected peak inductor current throughout the
normal operating range. The expected peak inductor
current is given by:
錚?/div>
I
MAIN(MAX)
V
MAIN
錚?錚?/div>
1
錚?/div>
I
PEAK
= 錚?/div>
錚鳳， 錚?+
V
IN(MIN)
錚?/div>
錚?錚?畏錚?/div>
錚?/div>
1
錚?錚?/div>
V
IN(MIN)
錚?錚?/div>
V
MAIN
- V
IN(MIN)
錚?/div>
錚?錚鳳，
錚?/div>
錚?/div>
2
錚?錚?/div>
V
MAIN
錚?錚?/div>
Lf
OSC
錚革－
錚?/div>
where
畏
is the efficiency of the regulator. For most
applications, the efficiency is between 75% and 85%.
Under fault conditions, the inductor current may reach
the internal LX current limit (see
Electrical Character-
istics).
However, soft saturation inductors and the con-
troller鈥檚 fast current-limit circuitry protect the device
from failure during such a fault condition.
The inductor鈥檚 DC resistance can significantly affect
efficiency due to the resistive power loss (P
LR
), which
can be approximated by the following equation:
錚?/div>
I
錚?/div>
脳
V
P
LR
=
I
LAVG
R
L
鈮?錚?/div>
MAIN MAIN
錚?/div>
R
L
V
IN
錚?/div>
錚?/div>
2
2
where I
LAVG
is the average inductor current and R
L
is
the inductor鈥檚 series resistance. For best performance,
select inductors with resistance less than the internal
N-channel MOSFET鈥檚 on-resistance (0.25鈩?typ). To
minimize radiated noise in sensitive applications, use a
shielded inductor.
21
______________________________________________________________________________________

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