Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
MAX685
Pin Description
PIN
1
2, 15
3
4
5
6
7
8
9
10
11
12
13, 14
16
NAME
LXP
I.C.
VP
POK
SEQ
SHDN
SYNC
V
DD
GND
FBN
REF
FBP
PGND
LXN
FUNCTION
P-Channel Switching Inductor Node. LXP turns off when the part enters shutdown.
Internally Connected. Do not externally connect.
Power Input. Connect to V
DD
.
Open-Drain Power-OK Output. POK is high when both outputs are in regulation. Connect POK to V
DD
with a
100kΩ pull-up resistor to V
DD
.
Power-Up Sequence Select Input. Connect SEQ to GND to power the negative output voltage first. Connect
SEQ to V
DD
to power the positive output first.
Shutdown Input. Both outputs go to 0V in shutdown. Connect to V
DD
for automatic startup.
Sync Input. This pin synchronizes the oscillator to an external clock frequency between 200kHz and 480kHz.
Connect SYNC to GND (220kHz) or V
DD
(400kHz) for internal oscillator frequency.
Supply Input. Bypass V
DD
with a 1.0µF or greater ceramic capacitor to GND.
Ground
Feedback Input for the Negative Output Voltage. Connect a resistor-divider between the negative output
and REF with the center to FBN to set the negative output voltage.
1.25V Reference Voltage Output. Bypass with 0.22µF to GND.
Feedback for the Positive Output Voltage. Connect a resistor-divider between the positive output and GND
with the center to FBP to set the positive output voltage.
Power Ground. Connect PGND to GND.
N-Channel Switching Inductor Node. LXN pulls to GND through the internal transistor when the part is shut
down.
_______________Detailed Description
The MAX685 DC-DC converter accepts an input voltage
between +2.7V and +5.5V and generates both a positive
and negative voltage, using a single inductor (Figure 1).
It alternates between acting as a step-up converter and
as an inverting converter on a cycle-by-cycle basis. Both
output voltages are independently regulated.
Each output is separately controlled by a pulse-width-
modulated (PWM) current mode regulator. This allows
the part to operate at a fixed frequency for use in noise-
sensitive applications. An internal oscillator runs at
220kHz or 400kHz, or can be synchronized to an exter-
nal signal. Since switching alternates between the two
regulators, each operates at half the oscillator frequency
(110kHz, 200kHz, or half the sync frequency). The oscil-
lator can be synchronized to a 200kHz to 480kHz clock.
On the first cycle of operation, the part operates as a
step-up converter. LXP connects to V
DD
, LXN pulls to
ground, and the inductor current rises. Once the induc-
6
tor current rises to a level set by the positive-side error
amplifier, LXN releases and the inductor current flows
through D2 to the positive output. When the inductor
current drops to zero (which happens each cycle under
normal, discontinuous operation), LXN returns to the
input voltage.
On the second cycle, LXN is held at ground. LXP is
pulled up to the input voltage until the current reaches
the limit set by the negative error amplifier. Then LXP is
released and the inductor current flows through D1 to
the negative output. Once the inductor current reaches
zero, the voltage at LXP returns to ground. The wave-
forms at LXN and LXP are shown in Figure 2 for a typi-
cal pair of cycles.
The current into the LXN pin is sensed to measure the
inductor current. The MAX685 controls the inductor cur-
rent to regulate both the positive and negative output
voltages.
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