Rainbow Electronics MAX17075 - Manual (Page 13)

Main Step-Up Regulator
The main step-up regulator employs a current-mode,
fixed-frequency PWM architecture to maximize loop
bandwidth and provide fast-transient response to
pulsed loads that are typical for TFT LCD panel source
drivers. The 1.2MHz switching frequency allows the use
of low-profile inductors and ceramic capacitors to mini-
mize the thickness of LCD panel design. The integrated
high-efficiency MOSFET and the built-in digital soft-start
function reduce the number of external components
required while controlling inrush currents. The output
voltage can be set from V
IN to 18V with an external
resistive voltage-divider. The regulator controls the out-
put voltage and the power delivered to the output by
modulating the duty cycle (D) of the internal power
MOSFET in each switching cycle. The duty cycle of the
MOSFET is approximated by:
where V AVDD is the output voltage of the step-up regulator.
Figure 3 shows the functional diagram of the step-up
regulator. An error amplifier compares the signal at FB
to 1.25V and changes the COMP output. The voltage at
COMP sets the peak inductor current. As the load
varies, the error amplifier sources or sinks current to the
COMP output accordingly to produce the inductor peak
current necessary to service the load. To maintain sta-
bility at high duty cycles, a slope-compensation signal
is summed with the current-sense signal.
D
V V
V
AVDD IN
AVDD
≈
−
MAX17075
Boost Regulator with Integrated Charge Pumps,
Switch Control, and High-Current Op Amp
______________________________________________________________________________________ 13
LOGIC
AND
DRIVER
SOFT-
START
I
LIMIT
CURRENT-LIMIT
COMPARATOR
PWM
COMPARATOR
ERROR AMP
LX
PGND
FB
CLOCK
SLOPE COMP
TO FAULT LOGIC
FAULT
COMPARATOR
1.0V
1.25V
COMP
CURRENT
SENSE
OSCILLATOR
∑
MAX17075
Figure 3. Step-Up Regulator Functional Diagram
DESIGNATION DESCRIPTION
C1, C2
10μF
±20%, 6.3V X5R ceramic capacitors
(0603)
Murata GRM188R60J106M
TDK C1608X5R0J106M
C3, C4, C7
10μF
±20%, 25V X5R ceramic capacitors
(1206)
Murata GRM31CR61E106M
TDK C3216X5R1E106M
C10, C14
1μF
±10%, 50V X7R ceramic capacitors
(1206)
Murata GRM31MR71H105KA
TDK C3216X7R1H105K
DESIGNATION DESCRIPTION
C11, C15, C16,
C17
0.1μF
±10%, 50V X7R ceramic capacitors
(0603)
Murata GRM188R71H104K
TDK C1608X7R1H104K
D1
3A, 30V Schottky diode (M-Flat)
Toshiba CMS02 (TE12L,Q) (Top mark S2)
D2, D3, D4
220mA, 100V dual diodes (SOT23)
Fairchild MMBD4148SE (Top mark D4)
L1
3.0μH, 3A DC inductor
Sumida CDRH6D28-3R0
Table 1. Component List
SUPPLIER PHONE FAX WEBSITE
Fairchild Semiconductor 408-822-2000 408-822-2102 www.fairchildsemi.com
Sumida 847-545-6700 847-545-6720 www.sumida.com
TDK 847-803-6100 847-390-4405 www.component.tdk.com
Toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec
Table 2. Component Suppliers