Datasheet
MAX1997/MAX1998
Quintuple/Triple-Output TFT LCD Power Supplies
with Fault Protection and VCOM Buffer
24 ______________________________________________________________________________________
Input P-Channel MOSFET
Select the input P-channel MOSFET based on current
rating, voltage rating, gate threshold, and on-resis-
tance. The MOSFET must be able to handle the peak
input current (see the Inductor Selection section). The
drain-to-source voltage rating of the input MOSFET
should be higher than the maximum input voltage.
Because the MOSFET conducts the full input current,
its on-resistance should be low enough for good effi-
ciency. Use a logic-level or low-threshold MOSFET to
ensure that the switch is fully enhanced at the lowest
input voltage.
Setting the Input Overcurrent Threshold
The high-side comparator of the MAX1997/MAX1998
provides input overcurrent protection when used in con-
junction with an external P-channel MOSFET P1. The
accuracy of the overcurrent threshold is affected by
many factors, including comparator offset, resistor toler-
ance, input voltage range, and variations in MOSFET
R
DS(ON)
. The input overcurrent comparator is only
intended to protect against catastrophic failures. This
function is similar to an input fuse.
To minimize the impact of the comparator’s input offset
on the current-sense accuracy, the sense voltage
should be close to the upper limit of the comparator’s
common-mode range (same as the operating range),
which extends up to 80% of the input voltage. The
resistive voltage divider R4/R5, combined with the on-
state resistance of P1, sets the overcurrent threshold.
The center of R4/R5 is connected to the inverting input
(OCN) as shown in Figure 10.
If the comparator and resistors are ideal, the threshold
is at the current where both inputs are equal:
I
L(MAX)
is the average inductor current at maximum load
condition and minimum input voltage, and is given by:
where η is the efficiency of the main step-up regulator.
If the step-up regulator’s minimum input voltage is 2.7V,
the output voltage is 9V, and the maximum load current is
0.3A. Assuming 80% efficiency, the maximum average
inductor current is:
R
DS(MAX)
is the maximum on-state drain-to-source
resistance of P1. The maximum R
DS(ON)
at +25°C can
be found in the MOSFET manufacturer’s data sheet, but
that number does not include the MOSFET’s tempera-
ture coefficient. Since the resistance temperature coef-
ficient is 0.5%/°C, R
DS(MAX)
can be calculated with the
following equation:
R
DS(MAX)
= R
DS_25C
✕
[1 + 0.005
✕
(T
J
- 25)]
where T
J
is the actual MOSFET junction temperature in
normal operation due to ambient temperature and self-
heating caused by power dissipation. As an example,
consider the Fairchild FDN304P, which has a maximum
R
DS(ON)
at room temperature of 70mΩ. If the junction
temperature is +100°C, the maximum on-state resis-
tance over temperature is:
R
DS(MAX)
= 70mΩ [1 + 0.005
✕
(100 - 25)] = 100mΩ
For given R2 and R3 values, the ideal ratio of R4/R5
can be determined:
R
R
RR
R
4
5
23
3
1=
+
×
×V - I R
V
-
IN PEAK(MAX) DS(MAX)
IN
I
9V
0.8 2.7V
0.3A = 25A
L(MAX)
=
×
×
.1
I
V
V
I
L(MAX)
OUT
IN(MIN)
LOAD(MAX)
=
×
×
η
VV-IR
IN IN L(MAX) DS(MAX)
×
+
=×
()
×
+
R
RR
R
RR
3
23
5
45
Figure 10. Setting the Overcurrent Threshold
V
IN
P1
R
DS(ON)
R5R3
R4R2
OC COMP
OCP
OCN
100pF