MAX685 Maxim, MAX685 Datasheet - Page 8

• Manufacturer Part Number: MAX685
• Description: Dual-Output Positive and Negative / DC-DC Converter for CCD and LCD
• Manufacturer: Maxim

Figure 3 shows the standard application circuit for the
MAX685. The values shown in Table 1 will work well for
output currents up to 10mA. However, this circuit can
be optimized to a particular application by using differ-
ent capacitors and a different inductor.
If the application requires output voltages greater than
-7.5V or +24V, use the circuit of Figure 4. This circuit
uses a charge pump to increase the output voltage
without increasing the voltage stress on the LX_ pin.
The maximum output voltages of the circuit in Figure 4
are -15V and +48V.
The voltage rating on D2, D5, and D6 must be 30V or
greater. For a larger negative output voltage without a
larger positive output (or vice versa), use one-half of
the Figure 4 circuit with one-half of the Figure 3 circuit.
A 22µH inductor is suitable for most applications.
Larger inductances will reduce inductor ripple current
and output voltage ripple, but they also typically require
larger physical size if increased resistance and losses
are not also allowed.
Dual-Output (Positive and Negative),
DC-DC Converter for CCD and LCD
Figure 3. Standard Application Circuit
8
V
IN
_______________________________________________________________________________________
0.22 F
2.2 F
V
C3
C1
10 F
-7.5V
C2
OUT-
NBR0520
R4
124k
R3
750k
Applications Information
D1
10
LXP
REF VP
FBN
1
11
MAX685
GND
Higher Output Voltages
3
9
22 H
L1
PGND LXN
V
DD
13, 14 16
8
SHDN
SYNC
POK 4
FBP
NBR0520
6
7
12
1.0M
D2
Inductor Selection
R1
R5
100k
C4
2.2 F
C5
47pF
+15V
V
OUT+
POK
SHDN
SYNC
R2
90.9k
Small inductors are typically preferred because of
compact design and low cost. Murata LHQ and
TDK NLC types are examples of small surface-mount
inductors that work for most applications. Because these
small-size inductors use thinner wire, they exhibit higher
resistance and have greater losses than larger ones. If
the application demands higher efficiency, use larger,
lower resistance coils such as the Sumida CD43 or CD54,
Coilcraft DT1608 or DO1608, or Coiltronics UP1V series.
The output ripple voltage is a function of the peak in-
ductor current, frequency, and type and value of the
output capacitors. Capacitors with low equivalent-
series resistance (ESR) and large capacitance reduce
output ripple. Typically, tantalum or ceramic capacitors
are optimal. Tantalum capacitors have higher ESR and
higher capacitance than ceramic capacitors. Therefore
the ESR of tantalum capacitors determines the output
ripple, because at the frequencies used the ESR domi-
nates the impedance of the capacitor. If ceramic
capacitors are used, the capacitance determines the
output ripple.
Figure 4. Circuit for Output Voltages < -9V and > +24V
V
2.2 F
OUT-
V
C7
0.22 F
IN
C2
R3
D3
C1
10 F
2.2 F
C3
D4
1 F
C6
R4
10
D1
LXP
REF VP
FBN
1
11
MAX685
GND
3
9
22 H
L1
PGND LXN
V
DD
13, 14 16 C8
Filter Capacitor Selection
8
SHDN
SYNC
POK 4
FBP
2.2 F
C4
D2
6
7
12
1 F
D5
2.2 F
R5
100k
R1
D6
C9
V
OUT+
C5
47pF
POK
SHDN
SYNC
R2