MAX1779 Maxim, MAX1779 Datasheet - Page 13

• Manufacturer Part Number: MAX1779
• Description: Low-Power Triple-Output TFT LCD DC-DC Converter
• Manufacturer: Maxim

where R
performance, select inductors with resistance less than
the internal N-channel MOSFET on-resistance (1Ω typ).
The output capacitor selection depends on circuit sta-
bility and output voltage ripple. In order to deliver the
maximum output current capability of the MAX1779, the
inductor must run in continuous-conduction mode (see
Inductor Selection). The minimum recommended output
capacitance is:
For configurations that need less output current, the
MAX1779 allows lower output capacitance when oper-
ating in discontinuous-conduction mode throughout the
load range. Under these conditions, at least 10µF is
recommended, as shown in Figure 6. In both discontin-
uous and continuous operation, additional feedback
compensation is required (see the Feedback
Compensation section) to increase the margin for sta-
bility by reducing the bandwidth further. In cases where
the output capacitance is sufficiently large, additional
feedback compensation will not be necessary.
However, in certain applications that require benign
load transients and constantly operate in discontinu-
ous-conduction mode, output capacitance less than
10µF may be used.
Output voltage ripple has two components: variations in
the charge stored in the output capacitor with each LX
pulse, and the voltage drop across the capacitor’s
equivalent series resistance (ESR) caused by the cur-
rent into and out of the capacitor:
For low-value ceramic capacitors, the output voltage
ripple is dominated by V
The MAX1779 contains an internal current integrator
that improves the DC load regulation but increases the
peak-to-peak transient voltage (see the Load Transient
Waveforms in the Typical Operating Characteristics).
For highly accurate DC load regulation, enable the inte-
grator by connecting a capacitor to INTG. The minimum
capacitor value should be C
is greater. Alternatively, to minimize the peak-to-peak
transient voltage at the expense of DC load regulation,
disable the integrator by connecting INTG to REF and
adding a 100kΩ resistor to GND.
L
V
is the inductor’s series resistance. For best
RIPPLE
C
OUT
Low-Power Triple-Output TFT LCD DC-DC
>
= V
______________________________________________________________________________________
60
V
RIPPLE(C)
MAIN
× ×
L I
RIPPLE(C)
×
MAIN MAX
OUT
V
IN MIN
+ V
(
(
/10k or 1nF, whichever
.
Integrator Capacitor
RIPPLE(ESR)
)
)
Output Capacitor
Compensation on the feedback node is required to
have enough margin for stability. Add a pole-zero pair
from FB to GND in the form of a compensation resistor
(R
tion capacitor (C
ous conduction operation, select R
value of R2, the low-side feedback resistor. For discon-
tinuous-conduction operation, select R
the value of R2.
Start with a compensation capacitor value of (220pF
R
stability as necessary. Larger compensation values
slow down the converter’s response time. Check the
startup waveform for excessive overshoot each time the
compensation capacitor value is increased.
The efficiency characteristics of the MAX1779 regulated
charge pumps are similar to a linear regulator. They are
dominated by quiescent current at low output currents
and by the input voltage at higher output currents (see
Typical Operating Characteristics). So the maximum
efficiency may be approximated by:
where N is the number of charge-pump stages.
Adjust the positive output voltage by connecting a volt-
age-divider from the output (V
Typical Operating Circuit). Adjust the negative output
voltage by connecting a voltage-divider from the output
(V
100kΩ range. Higher resistor values improve efficiency
at low output current but increase output voltage error
due to the feedback input bias current. Calculate the
remaining resistors with the following equations:
where V
+40V, and V
Increasing the flying capacitor’s value increases the
output current capability. Above a certain point,
increasing the capacitance has a negligible effect
because the output current capability becomes domi-
COMP
NEG
COMP
) to FBN to REF. Select R4 and R6 in the 50kΩ to
)/10kΩ. Increase this value to improve the DC
REF
in Figures 5 and 6) in series with a compensa-
Efficiency ≅ V
NEG
Efficiency ≅
for the negative charge pump
= 1.25V. V
for the positive charge pump
R3 = R4 [(V
R5 = R6 (
COMP
may range from 0 to -40V.
in Figures 5 and 6). For continu-
I
POS
POS
I
V
V
POS
NEG
NEG
Efficiency Considerations
/ [V
may range from V
Output Voltage Selection
Feedback Compensation
POS
/ V
/ V
I
Converter
IN
/ [V
REF
REF
) to FBP to GND (see
✕
COMP
IN
(N + 1)];
) - 1]
I
)
✕
Flying Capacitor
Charge Pump
COMP
N];
to be 1/2 the
to be 1/5th
SUPP
13
to
✕