Rainbow Electronics MAX1747 - Manual (Page 11)

MAX1747
Triple Charge-Pump TFT LCD
DC-DC Converter
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in the Typical Operating Characteristics). The main out-
put voltage is controlled to be in regulation within 4096
clock cycles (1/f
OSC ). The negative and positive low-
power charge pumps are controlled to be in regulation
within 2048 clock cycles.
Power-Up Sequencing
Upon power-up or exiting shutdown, the MAX1747
starts a power-up sequence. First, the reference pow-
ers up. Then the primary charge pump powers up with
soft-start enabled. Once the main charge pump reach-
es 90% of its nominal value (V
FB
> 1.125V), the nega-
tive charge pump turns on. When the negative output
voltage reaches approximately 90% of its nominal value
(V
FBN < 125mV), the positive charge pump starts up.
Finally, when the positive output voltage reaches 90%
of its nominal value (V
FBP > 1.125V), the active-low
ready signal (RDY) goes low (see Power Ready).
Fault Detection
Once RDY is low, and if any output falls below its fault
detection threshold, RDY goes high impedance.
For the reference, the fault threshold is 1.05V. For the
main charge pump, the fault threshold is 88% of its
nominal value (V
FB < 1.1V). For the negative charge
pump, the fault threshold is approximately 88% of its
nominal value (V
FBN > 140mV). For the positive charge
pump, the fault threshold is 88% of its nominal value
(V
FBP < 1.1V).
Once an output faults, all outputs later in the power
sequence shut down until the faulted output rises
above its power-up threshold. For example, if the nega-
tive charge-pump output voltage falls below the fault-
detection threshold, the main charge pump remains
active while the positive charge pump stops switching
and its output voltage decays, depending on output
capacitance and load. The positive charge-pump out-
put will not power up until the negative charge-pump
output voltage rises above its power-up threshold (see
Power-Up Sequencing).
Power Ready
Power ready is an open-drain output. When the power-
up sequence is properly completed, the MOSFET turns
on and pulls RDY low with a typical 125
Ω R
ON . If a fault
is detected, the internal open-drain MOSFET appears
as a high impedance. Connect a 100k
Ω pullup resistor
between RDY and IN for a logic level output.
Voltage Reference
The voltage at REF is nominally 1.25V. The reference
can source up to 50mA with excellent load regulation
(see Typical Operating Characteristics). Connect a
0.22µF bypass capacitor between REF and GND.
During shutdown, the reference is disabled.
Design Procedure
Efficiency Considerations
The efficiency characteristics of the MAX1747 regulat-
ed charge pumps are similar to a linear regulator. They
are dominated by quiescent current at low output cur-
rents and by the input voltage at higher output currents
(see Typical Operating Characteristics). Therefore, the
maximum efficiency may be approximated by:
Efficiency ≅ V
OUT / (2 ✕ V SUPM ) for the main
charge pump
Efficiency ≅ - V
NEG / (V SUPN
✕ N) for the negative
low-power charge pump
OSC
SUPP
DRVP
FBP
PGND
V
REF
1.25V
MAX1747
GND
C3
D1
D2
R3
R4
C4
V POS
V
POS
= [1 + (R3/R4)] ✕ V
REF
V
REF
= 1.25V
V SUPP = 2.7V TO 13V
Figure 4. Positive Charge-Pump Block Diagram
MAX1747 IN
SHDN
GND
R FREQ
V ON = V IN
C IN
R FREQ = k FREQ ✕ (V ON - 0.7V)/f OSC
R FREQ IS IN k Ω, k
FREQ IS 45.5MHz/mA,
AND f OSC IS IN MHz.
OSC
Figure 5. Frequency Adjustment