MAX8740 Maxim Integrated Products, MAX8740 Datasheet - Page 10

• Manufacturer Part Number: MAX8740
• Description: TFT-LCD Step-Up DC-DC Converter
• Manufacturer: Maxim Integrated Products

TFT-LCD Step-Up DC-DC Converter
Figure 3. Multiple-Output TFT-LCD Power Supply
Figure 3 shows a power supply for active-matrix TFT-
LCD flat-panel displays. Output-voltage transient perfor-
mance is a function of the load characteristic. Add or
remove output capacitance (and recalculate compensa-
tion-network component values) as necessary to meet
the required transient performance. Regulation perfor-
mance for secondary outputs (V2 and V3) depends on
the load characteristics of all three outputs.
Careful PC board layout is important for proper operation.
Use the following guidelines for good PC board layout:
1) Minimize the area of high-current loops by placing
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Multiple-Output Power Supply for TFT LCD
the inductor, rectifier diode, and output capacitors
near the input capacitors and near the LX and GND
pins. The high-current input loop goes from the
positive terminal of the input capacitor to the induc-
tor, to the IC’s LX pin, out of GND, and to the input
capacitor’s negative terminal. The high-current out-
put loop is from the positive terminal of the input
capacitor to the inductor, to the rectifier diode (D1),
______________________________________________________________________________________
PC Board Layout and Grounding
4.5V TO 5.5V
V
IN
10µF
6.3V
C1
+28V C9
V2
1µF
10Ω
1µF
C5
R4
33nF
C4
10
8
9
3
D2
IN
FREQ
SHDN
SS
2.7µH
L1
LX
6
0.1µF
C7
MAX8740
0.1µF
C8
LX
COMP
7
GND
GND
D1
FB
D3
2
5
4
1
2) Create a power ground island (PGND) consisting of
and to the positive terminal of the output capacitors,
reconnecting between the output capacitor and
input capacitor ground terminals. Connect these
loop components with short, wide connections.
Avoid using vias in the high-current paths. If vias
are unavoidable, use many vias in parallel to
reduce resistance and inductance.
the input and output capacitor grounds and GND
pins. Connect all of these together with short, wide
traces or a small ground plane. Maximizing the
width of the power ground traces improves efficien-
cy and reduces output voltage ripple and noise
spikes. Create an analog ground plane (AGND)
consisting of the feedback-divider ground connec-
tion, the COMP and SS capacitor ground connec-
tions, and the device’s exposed backside pad.
Connect the AGND and PGND islands by connect-
ing the GND pins directly to the exposed backside
pad. Make no other connections between these
separate ground planes.
R3
47kΩ
1%
C3
560pF
R1
196kΩ
1%
R2
20kΩ
1%
C10
0.22µF
C6
68pF
V3
-10V
C2
10µF
25V
C7
10µF
25V
V
13.5V/800mA
OUT