MAX1997ETJ Maxim Integrated Products, MAX1997ETJ Datasheet - Page 24

• Manufacturer Part Number: MAX1997ETJ
• Description: LCD Drivers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1997ETJ

• Dc: 0618
• Lead Free Status / Rohs Status: No

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.
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
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.
Quintuple/Triple-Output TFT LCD Power Supplies
with Fault Protection and VCOM Buffer
Figure 10. Setting the Overcurrent Threshold
24
DS(ON)
______________________________________________________________________________________
V
IN
. The input overcurrent comparator is only
R2
R3
Setting the Input Overcurrent Threshold
R
DS(ON)
P1
100pF
R4
R5
Input P-Channel MOSFET
OCP
OCN
OC COMP
If the comparator and resistors are ideal, the threshold
is at the current where both inputs are equal:
I
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
resistance of P1. The maximum R
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
following equation:
where T
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
temperature is +100°C, the maximum on-state resis-
tance over temperature is:
For given R2 and R3 values, the ideal ratio of R4/R5
can be determined:
L(MAX)
DS(ON)
DS(MAX)
R
V
DS(MAX)
IN
R
R
R
×
DS(MAX)
4
5
is the average inductor current at maximum load
J
R
at room temperature of 70mΩ. If the junction
2
=
is the actual MOSFET junction temperature in
R
I
is the maximum on-state drain-to-source
+
L(MAX)
I
R
= 70mΩ [1 + 0.005
3
L(MAX)
R
2
R
3
+
= R
3
=
R
3
(
=
DS_25C
V - I
=
×
IN
DS(MAX)
0.8
η
V
×
IN
L(MAX)
V
V
9V
×
OUT
IN(MIN)
- I
✕
2.7V
PEAK(MAX)
[1 + 0.005
can be calculated with the
×
✕
×
R
×
(100 - 25)] = 100mΩ
DS(ON)
DS(MAX)
I
V
LOAD(MAX)
0.3A = 25A
IN
✕
×
R
(T
. 1
at +25°C can
)
DS(MAX)
J
×
- 25)]
R
4
R
+
5
R
-
5
1