MAX1889ETE Maxim Integrated Products, MAX1889ETE Datasheet - Page 19

• Manufacturer Part Number: MAX1889ETE
• Description: DC/DC Switching Converters
• Manufacturer: Maxim Integrated Products

Specifications of MAX1889ETE

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The input capacitor (C
drawn from the input supply and reduces noise injection
into the device. Two 3.3µF ceramic capacitors are used
in the standard application circuit (Figure 1) because of
the high source impedance seen in typical lab setups.
Actual applications usually have much lower source
impedance since the step-up regulator typically runs
directly from the output of another regulated supply.
Typically, C
the standard applications circuit. Ensure a low noise sup-
ply at the IN pin by using adequate C
greater voltage variation can be tolerated on C
decoupled from C
C1 in Figure 1).
The MAX1889’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. In general, a 1A Schottky
diode complements the internal MOSFET well.
Select the input P-channel MOSFET based on the cur-
rent 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,
the on-resistance should be low enough for higher effi-
ciency. Use a low-threshold MOSFET to ensure that the
switch is fully enhanced at lowest input voltages.
The high-side comparator of the MAX1889 provides
input overcurrent protection when used in conjunction
with an external P-channel MOSFET P1. The accuracy
of the overcurrent threshold is affected by many fac-
tors, including comparator offset, resistor tolerance,
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 common-mode
range, which extends up to 80% of the input voltage.
The resistive voltage-divider (R3/R4), combined with
the on-state resistance of P1, sets the overcurrent
threshold. The center of R3/R4 is connected to the
inverting input (OCN) as shown in Figure 6.
DS(ON)
. The input overcurrent comparator is only
IN
Setting the Input Overcurrent Threshold
can be reduced below the values used in
IN
______________________________________________________________________________________
using an RC lowpass filter (see R1,
IN
) reduces the current peaks
Triple-Output TFT LCD Power Supply
Input P-Channel MOSFET
Input Capacitor
IN
Rectifier Diode
. Alternatively,
IN
if IN is
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 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,
output voltage is 9V and maximum load current is 0.3A.
Assuming 80% efficiency, the maximum average induc-
tor current is:
R
resistance of P1. The maximum R
be found in the MOSFET data sheet, but that number
does not include the temperature coefficient.
Since the temperature coefficient for the resistance is
0.5%/°C, R
equation:
where T
normal operation due to ambient temperature rise and
self-heating caused by power dissipation. As an exam-
ple, consider Fairchild FDN304P, which has a maxi-
mum R
Figure 6. Setting the Overcurrent Threshold
L(MAX)
DS(MAX)
V
with Fault Protection
IN
R
×
DS(ON)
is the average inductor current at maximum load
DS MAX
J
R
1
is the actual MOSFET junction temperature in
(
R
DS(MAX)
is the maximum on-state drain-to-source
+
I
L MAX
2
I
L MAX
R
(
(
2
V
at room temperature of 70mΩ.
)
IN
=
=
)
(
R
)
V
=
IN L MAX
DS
can be calculated with the following
=
η
R1
R2
-
0 8 2 7
R
_
I
×
DS(ON)
.
25
V
(
V
OUT
9
×
IN MIN
°
C
V
(
.
× +
)
[
V
×
1 0 005
R3
R4
)
R
×
OCN
OCP
DS MAX
×
0 3
DS(ON)
I
.
.
LOAD MAX
(
A
=
×
(
)
1 25
(
)
T
.
at +25°C can
×
OC COMP
J
R
-
)
A
25
3
R
+
)
4
]
R
4
19