MAX1889ETE Maxim Integrated Products, MAX1889ETE Datasheet - Page 21

• 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 number of negative charge-pump stages is given by:
where N
stages, V
V
forward voltage drop of the charge-pump diode, and
V
Use V
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to V
negative charge pump is connected to ground.
Sometimes fractional stages are more desirable for bet-
ter efficiency. This can be done by connecting the first
stage to V
If the first charge-pump stage is powered from V
then the above equations become:
Increasing the flying capacitor (C
output current capability. Increasing the capacitance
indefinitely has a negligible effect on output current
capability because the internal switch resistance and
the diode impedance limit the source impedance. A
0.1µF ceramic capacitor works well in most low-current
applications. The flying capacitor’s voltage rating must
exceed the following:
where N is the stage number in which the flying capaci-
tor appears, and V
example, the two-stage positive charge pump in the
typical application circuit (Figure 1) where V
contains two flying capacitors. The flying capacitor in
the first stage (C14) requires a voltage rating over 9V.
The flying capacitor in the second stage (C13) requires
a voltage rating over 18V.
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to-
peak transient voltage. With ceramic capacitors, the
MAIN
DROPOUT
DROPOUT
is the main step-up regulator output, V
NEG
IN
NL
is the dropout margin for the linear regulator.
or another available supply.
is the negative linear-regulator output,
is the number of negative charge-pump
N
N
POS
NEG
= 2V.
N
NEG
______________________________________________________________________________________
MAIN
=
V
=
CX
Charge-Pump Output Capacitor
V
-
=
V
PL
-
NL
MAIN
is the main output voltage. For
> ×
V
V
V
NL
+
N V
MAIN
MAIN
V
+
Triple-Output TFT LCD Power Supply
V
MAIN
DROPOUT
V
+
and the first stage of the
DROPOUT
MAIN
V
DOPOUT
- 2
-
X
-
2
) value increases the
×
2
×
V
×
V
D
Flying Capacitor
D
V
-
D
V
+
IN
V
IN
MAIN
D
is the
= 9V
IN
,
output voltage ripple is dominated by the capacitance
value. Use the following equation to approximate the
required capacitor value:
where V
ripple.
Use Schottky diodes with a current rating equal to or
greater than two times the average charge-pump input
current.
Adjust the positive linear-regulator output voltage by
connecting a resistive voltage-divider from V
with the center tap connected to FBP (Figure 1). Select
R13 in the range of 10kΩ to 30kΩ.
Calculate R12 with the following equation:
where V
Adjust the negative linear-regulator output voltage by
connecting a resistive voltage-divider from V
with the center tap connected to FBN (Figure 1). Select
R10 in the range of 10kΩ to 30kΩ. Calculate R9 with the
following equation:
where V
only guaranteed to source 50µA. Using a resistor less
than 20kΩ for R10 results in higher bias current than
REF can supply. Connecting another resistor (R14)
from V
because the main output can supply part of the resis-
tor’s (R10) bias current. Use the following equation to
determine the value of R14:
Drawing only 40µA from REF leaves the remaining
10µA for other purposes.
The pass transistor must meet specifications for current
gain (β), input capacitance, collector-emitter saturation
voltage, and power dissipation.
with Fault Protection
MAIN
FBP
R9 = R10 [(V
RIPPLE
FBN
= 1.25V.
to REF (Figure 1) can solve this problem
= 125mV, V
R12 = R13 [(V
R
14
is the peak-to-peak value of the output
C
OUT
=
Linear-Regulator Controllers



FBN
V
≥
REF
Charge-Pump Rectifier Diodes
2
V
REF
f
- V
R
MAIN
OSC RIPPLE
10
-
V
I
PL
NL
Pass Transistor Selection
LOAD
Output Voltage Selection
FBN
= 1.25V. Note that REF is
V
) / (V
/ V
-
V
REF



FBP
-
REF
40
) - 1]
µ
- V
A
FBN
PL
NL
)]
to GND
to REF
21