Rainbow Electronics MAX1779 - Manual (Page 13)

MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
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where R L is the inductor’s series resistance. For best
performance, select inductors with resistance less than
the internal N-channel MOSFET on-resistance (1
Ω typ).
Output Capacitor
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:
V RIPPLE = V RIPPLE(C) + V RIPPLE(ESR)
For low-value ceramic capacitors, the output voltage
ripple is dominated by V
RIPPLE(C) .
Integrator Capacitor
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
OUT /10k or 1nF, whichever
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 100k
Ω resistor to GND.
Feedback Compensation
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
COMP in Figures 5 and 6) in series with a compensa-
tion capacitor (C COMP in Figures 5 and 6). For continu-
ous conduction operation, select R COMP to be 1/2 the
value of R2, the low-side feedback resistor. For discon-
tinuous-conduction operation, select R
COMP to be 1/5th
the value of R2.
Start with a compensation capacitor value of (220pF ✕
R COMP )/10k
Ω. Increase this value to improve the DC
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.
Charge Pump
Efficiency Considerations
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:
Efficiency ≅ I V NEG
I / [V IN
✕ N];
for the negative charge pump
Efficiency
≅ V
POS / [V IN
✕ (N + 1)];
for the positive charge pump
where N is the number of charge-pump stages.
Output Voltage Selection
Adjust the positive output voltage by connecting a volt-
age-divider from the output (V
POS ) to FBP to GND (see
Typical Operating Circuit). Adjust the negative output
voltage by connecting a voltage-divider from the output
(V
NEG ) to FBN to REF. Select R4 and R6 in the 50k Ω to
100k
Ω 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:
R3 = R4 [(V POS / V REF ) - 1]
R5 = R6 ( I V NEG / V REF
I )
where V REF = 1.25V. V POS may range from V SUPP to
+40V, and V NEG may range from 0 to -40V.
Flying Capacitor
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-
C
L I
V V
OUT
MAIN MAX
MAIN IN MIN
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