Rainbow Electronics MAX1621 - Manual (Page 10)

MAX1620/MAX1621
Digitally Adjustable LCD Bias Supplies
10 ______________________________________________________________________________________
R1
360k
2V
TO
12V
BATT
POK
D1
MBRS0540
N1
MMFT3055VL Q1
MMBT2907
V DD
POL
SHDN (SUS)
DN (SDA)
UP (SCL)
REF
AGND
3
5
OPTIONAL
11
7
4
1
2
6
12
( ) ARE FOR MAX1621.
NOTE: CONNECTIONS TO DIGITAL INPUTS NOT SHOWN.
14
16
15
13
R3
300k
R4
300k
R5
2.2M
C6
100pF
10
9
8
LX
DHI
DLO
PGND
DOUT
FB
LCDON
3V
TO
5.5V
C1
0.1
µ F
C2
0.1
µ F
R2
100k
R8
10k
TO REF
D3 1N6263 (ANY SCHOTTKY)
C3
22
µ F
C5
22
µ F
12.5V
TO
23.5V OUT
VOUTSW
OPTIONAL
R6
56k
R7
56k
L1
100
µ H
MAX1620
MAX1621
U1
_______________Detailed Description
The MAX1620/MAX1621 are step-up power controllers
that drive an external N-channel FET or NPN transistor
to convert power from a 1.8V to 20V battery to a higher
positive or negative voltage. They are configured as
negative-output, inverting power controllers with one
additional diode and one additional capacitor. Either
configuration’s output voltage can be adjusted with
external resistors, or digitally adjusted with an internal
digital-to-analog converter (DAC). The MAX1620 uses
pin-defined controls for the DAC, while the MAX1621
communicates with the DAC via the SMBus™ interface.
Operating Principle
The MAX1620/MAX1621 operate in discontinuous-
conduction mode (where the inductor current ramps to
zero by the end of each switching cycle) and with a
constant peak current, without requiring a current-
sense resistor. Switch on-time is inversely proportional
to the input voltage V
BATT by a microsecond-volt con-
stant, or k-factor, of 20µs-V (e.g., for V BATT = 10V,
on-time = 2µs).
For an ideal boost converter operating in discontinu-
ous-conduction mode (no power losses), output current
is proportional to input voltage and peak inductor current:
I PK is proportional to on-time (t ON ), which, for these
parts, is determined by the k-factor:
I PK = k-factor / L
Discontinuous conduction is detected by monitoring the
LX node voltage. When the inductor’s energy is com-
pletely delivered, the LX node voltage snaps back to
the BATT voltage. When this crossing is sensed, anoth-
er pulse is issued if the output is still out of regulation.
Positive Output Voltage
To select a positive output voltage, tie the polarity pin
(POL) to V
DD and use the typical boost topology shown
in Figure 4. FB regulation voltage is 1.5V. For optimum
stability, V
OUT should be greater than 1.1 (V BATT ).
Negative Output Voltage
To select a negative output voltage, tie POL to GND
(Figure 5). In this configuration, the internal error amplifi-
er’s output is inverted to provide the correct feedback
polarity. FB regulation voltage is 0V. D1, D2, C4, and C5
form an inverting charge pump to generate the negative
voltage. This allows application of the positive boost
switching topology to negative output voltages.
The negative output circuit has two possible connec-
tions. In the standard connection, D1’s cathode is con-
nected to BATT. This connection features the best
output ripple performance, but

V OUT

must be limited
to no more than 27V - 1.1(V BATT ). If a larger negative
voltage is needed, an alternative connection allows a
maximum negative output of -27V, but with the addition-
al constraint that

V OUT

> 1.1V BATT . To use the alter-
native circuit, connect D1’s cathode to ground rather
than BATT (Figure 6). Increase C4 to 2.2
µ
F to improve
output ripple performance.
The negative charge pump limits the output current to
the charge transferred each cycle multiplied by the
I
1
2
I V / V
OUT PK BATT OUT
= Ч Ч
Figure 4. Typical Operating Circuit—Positive Output