Design procedure – Rainbow Electronics MAX1779 User Manual
Page 12
MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
12
______________________________________________________________________________________
1.125V), the active-low ready signal (RDY) is pulled low
(see Power Ready section).
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 on-resis-
tance. 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.
Fault Detection
Once RDY is low, if any output falls below its fault-
detection threshold, then RDY becomes high imped-
ance.
For the reference, the fault threshold is 1.05V. For the
main boost converter, 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
< 1.11V).
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 boost converter 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
the Power-Up Sequencing section).
Voltage Reference
The voltage at REF is nominally 1.25V. The reference
can source up to 50µA with good load regulation (see
Typical Operating Characteristics). Connect a 0.22µF
bypass capacitor between REF and GND.
Design Procedure
Main Boost Converter
Inductor Selection
Inductor selection depends upon the minimum required
inductance value, saturation rating, series resistance,
and size. These factors influence the converter’s effi-
ciency, maximum output load capability, transient
response time, and output voltage ripple. For most
applications, values between 10µH and 33µH work
best with the controller’s switching frequency.
The inductor value depends on the maximum output
load the application must support, input voltage, and
output voltage. With high inductor values, the MAX1779
sources higher output currents, has less output ripple,
and enters continuous-conduction operation with lighter
loads; however, the circuit’s transient response time is
slower. On the other hand, low-value inductors respond
faster to transients, remain in discontinuous-conduction
operation, and typically offer smaller physical size. The
maximum output current an inductor value will support
may be calculated by the following equations:
A. Continuous-conduction: if
then
B. Discontinuous-conduction: if
then
where I
LIM(MIN)
= 350mA and ƒ = 250kHz (see the
Electrical Characteristics).
The inductor’s saturation current rating should exceed
peak inductor current throughout the normal operating
range. Under fault conditions, the inductor current may
reach up to 600mA (I
LIM(MAX)
, see the Electrical
Characteristics). However, the MAX1779’s fast current-
limit circuitry allows the use of soft-saturation inductors
while still protecting the IC.
The inductor’s DC resistance significantly affects effi-
ciency due to the power loss in the inductor. The power
loss due to the inductor’s series resistance (P
LR
) may
be approximated by the following equation:
P
I
V
V
R
LR
MAIN
MAIN
IN
L
≅
Ч
Ч
2
L
I
V
V
I
MAIN MAX
MAIN
IN MIN
LIM MIN
≥
ƒ
(
)
2
1
2
(
)
(
)
(
)
-
I
V
V
I
MAIN MAX
IN MIN
MAIN
LIM MIN
(
)
(
)
(
)
<
1
2
L
V
V
IN MIN
MAIN
V
V
V
I
I
MAIN VIN MIN
IN MIN
MAIN
LIM MIN
MAIN MAX
≥
ƒ
1
2
1
2
(
)
(
)
(
)
(
)
(
)
-
-
I
V
V
I
MAIN MAX
IN MIN
MAIN
LIM MIN
(
)
(
)
(
)
≥
1
2