MAX8798AETX+T Maxim Integrated Products, MAX8798AETX+T Datasheet - Page 18

• Manufacturer Part Number: MAX8798AETX+T
• Description: Display Drivers Internal-Switch Boos t Regulator with Int
• Manufacturer: Maxim Integrated Products

Specifications of MAX8798AETX+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Internal-Switch Boost Regulator with
Integrated 3-Channel Scan Driver for TFT LCDs
Figure 4 shows the block diagram of the step-up regu-
lator. An error amplifier compares the signal at FB to
1.24V and changes the COMP output. The voltage at
COMP determines the current trip point each time the
internal MOSFET turns on. As the load varies, the error
amplifier sources or sinks current to the COMP output
accordingly to produce the inductor peak current nec-
essary to service the load. To maintain stability at high
duty cycles, a slope-compensation signal is summed
with the current-sense signal.
On the rising edge of the internal clock, the controller
sets a flip-flop, turning on the n-channel MOSFET and
applying the input voltage across the inductor. The cur-
rent through the inductor ramps up linearly, storing
energy in its magnetic field. Once the sum of the cur-
rent-feedback signal and the slope compensation
exceed the COMP voltage, the controller resets the flip-
flop and turns off the MOSFET. Since the inductor cur-
rent is continuous, a transverse potential develops
across the inductor that turns on the diode (D1). The
Figure 4. Step-Up Regulator Block Diagram
18
______________________________________________________________________________________
OSCILLATOR
1.2MHz
CLOCK
TO FAULT LOGIC
COMPARATOR
PWM
SLOPE COMP
COMPARATOR
FAULT
COMPARATOR
ILIM
LOGIC AND DRIVER
1.0V
SOFT-
START
voltage across the inductor then becomes the differ-
ence between the output voltage and the input voltage.
This discharge condition forces the current through the
inductor to ramp back down, transferring the energy
stored in the magnetic field to the output capacitor and
the load. The MOSFET remains off for the rest of the
clock cycle.
The undervoltage lockout (UVLO) circuit compares the
input voltage at IN with the UVLO threshold (1.3V rising
and 1.2V falling) to ensure that the input voltage is high
enough for reliable operation. The 100mV (typ) hystere-
sis prevents supply transients from causing a restart.
Once the input voltage exceeds the UVLO rising
threshold, startup begins. When the input voltage falls
below the UVLO falling threshold, the controller turns
off the main step-up regulator and the linear regulator,
disables the switch-control block, and the operational
amplifier output becomes high impedance.
I
LIMIT
ERROR AMP
CURRENT
SENSE
1.24V
Undervoltage Lockout (UVLO)
PGND
FB
COMP
LX