Rainbow Electronics MAX8728 - Manual (Page 15)

MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
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Typical Operating Circuit
The typical operating circuit (Figure 1) of the MAX8728 is
a complete power-supply system for TFT LCD panels in
monitors and TVs. The circuit generates a +3.3V logic
supply, a +13.5V source driver supply, a +28V positive
gate driver supply, and a -6V negative gate driver supply
from a 12V ±10% input supply and operates at 1.5MHz.
Table 1 lists some selected components and Table 2
lists the contact information for component suppliers.
Detailed Description
The MAX8728 is a multiple-output power supply
designed primarily for TFT LCD panels used in monitors
and TVs. It contains a step-down switching regulator to
generate the logic supply rail, a step-up switching regu-
lator to generate the source driver supply, and two
charge-pump regulators to generate the gate-driver
supplies. Each regulator features adjustable output volt-
age, digital soft-start, and timer-delayed fault protection.
Both the step-down and step-up regulators use fixed-
frequency current-mode control architectures. The two
switching regulators are 180° out of phase to minimize
the input ripple. The internal oscillator offers three pin-
selectable frequency options (500kHz/1MHz/1.5MHz)
allowing users to optimize their designs based on the
specific application requirements. In addition, the
MAX8728 features a high-voltage switch-control block,
an internal 5V linear regulator, a 2V reference output,
well-defined power-up and power-down sequences,
and thermal-overload protection. Figure 2 shows the
MAX8728 functional diagram.
Step-Down Regulator
The step-down regulator consists of an internal n-chan-
nel MOSFET with gate driver, a lossless current-sense
network, a current-limit comparator, and a PWM con-
troller block. The external power stage consists of a
Schottky diode rectifier, an inductor, and output capac-
itors. The output voltage is regulated by changing the
duty cycle of the high-side MOSFET. A bootstrap circuit
that uses a 0.1µF flying capacitor between LX1 and
BST provides the supply voltage for the high-side gate
driver. Although the MAX8728 also includes a 25
Ω (typ)
low-side MOSFET, this switch is used to charge the
bootstrap capacitor during startup and maintains fixed-
frequency operation at light load and cannot be used
as a synchronous rectifier. An external Schottky diode
(D2 in Figure 1) is always required.
PWM Controller Block
The heart of the PWM control block is a multi-input,
open-loop comparator that sums three signals: the out-
put voltage signal with respect to the reference voltage,
the current-sense signal, and the slope compensation.
The PWM controller is a direct-summing type, lacking a
traditional error amplifier and the phase shift associated
with it. This direct-summing configuration approaches
ideal cycle-by-cycle control over the output voltage.
When EN is high or floating, the controller always oper-
ates in fixed-frequency PWM mode. Each pulse from
the oscillator sets the main PWM latch that turns on the
high-side switch until the PWM comparator changes
state. As the high-side switch turns off, the low-side
switch turns on. The low-side switch stays on until the
beginning of the next clock cycle.
When EN is low, the controller operates in skip mode.
The skip mode dramatically improves light-load effi-
ciency by reducing the effective frequency, which
reduces switching losses. It keeps the actual peak
inductor current at about 0.8A in an active cycle, allow-
ing subsequent cycles to be skipped. Skip mode tran-
sitions seamlessly to fixed-frequency PWM operation as
load current increases.
Table 1. Component List (1.5MHz)
DESIGNATION DESCRIPTION
C1, C3, C4, C5, C6
10µF ±20%, 16V X5R ceramic
capacitors (1206)
TDK C3216X5R1C106M
D1, D2
3A, 30V Schottky diode (M-flat)
Toshiba CMS02 (top mark S2)
D3, D4, D5
220mA, 100V dual diode (SOT23)
Fairchild MMBD4148SE (top mark D4)
L1
6.4µH, 1.5ADC inductor
Sumida CDRH6D12-6R4
L2
2.6µH, 2.6ADC inductor
Sumida CDRH6D12-2R6
P1
2.4A, -20V p-channel MOSFET
(3-pin SuperSOT)
Fairchild FDN304P (top mark 304)
Table 2. Component Suppliers
SUPPLIER PHONE
Fairchild Semiconductor 408-822-2000
Sumida 847-545-6700
TDK 847-803-6100
Toshiba 949-455-2000