MAX8758ETG+T Maxim Integrated Products, MAX8758ETG+T Datasheet - Page 13

• Manufacturer Part Number: MAX8758ETG+T
• Description: IC REG STEP UP 24-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8758ETG+T

• Applications: Converter, TFT, LCD
• Voltage - Input: 1.8 ~ 5.5 V
• Number Of Outputs: 1
• Voltage - Output: 5V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The soft-start timing can be adjusted with an external
capacitor connected between SS and GND. After the
step-up regulator is enabled, the SS pin is immediately
charged to 0.5V. Then the capacitor is charged at a
constant current of 4µA (typ). During this time, the SS
voltage directly controls the peak inductor current,
allowing a linear ramp from zero up to the full current
limit. The maximum load current is available after the
voltage on SS exceeds 1.5V. The soft-start capacitor is
discharged to ground when SHDN is low. The soft-start
routine minimizes inrush current and voltage overshoot
and ensures a well-defined startup behavior (see the
Step-Up Regulator Heavy Load Soft-Start waveform in
the Typical Operating Characteristics).
During steady-state operation, the MAX8758 monitors the
FB voltage. If the FB voltage is below 1V (typ), the
MAX8758 activates an internal fault timer. If there is a
continuous fault for the fault-timer duration, the MAX8758
sets the fault latch, shutting down all the outputs. Once
the fault condition is removed, cycle the input voltage to
clear the fault latch and reactivate the device. The fault-
detection circuit is disabled during the soft-start time.
The MAX8758 monitors the OUT voltage for undervoltage
and overvoltage conditions. If the OUT voltage is below
1.4V (typ) or above 13.5V (typ), the MAX8758 disables
the gate driver of the step-up regulator and prevents the
internal MOSFET from switching. The OUT undervoltage
and overvoltage conditions do not set the fault latch.
The thermal-overload protection prevents excessive
power dissipation from overheating the MAX8758.
When the junction temperature exceeds T
thermal sensor immediately activates the fault protec-
tion, which sets the fault latch and shuts down all the
outputs, allowing the device to cool down. Once the
device cools down by approximately 15°C, cycle the
input voltage or toggle SHDN to clear the fault latch
and restart the device.
The thermal-overload protection protects the controller
in the event of fault conditions. For continuous opera-
tion, do not exceed the absolute maximum junction
temperature rating of T
The FREQ pin selects the switching frequency. Table 3
shows the switching frequency based on the FREQ con-
nection. High-frequency (1.2MHz) operation optimizes
the application for the smallest component size, trading
off efficiency due to higher switching losses. Low-fre-
quency (600kHz) operation offers the best overall efficien-
cy at the expense of component size and board space.
Thermal-Overload Protection
______________________________________________________________________________________
Step-Up Regulator with Switch Control
J
and Operational Amplifier for TFT LCD
= +150°C.
Frequency Selection (FREQ)
Fault Protection
J
= +160°C, a
The MAX8758’s operational amplifier is typically used
to drive the LCD backplane (VCOM) or the gamma-cor-
rection-divider string. The operational amplifier features
±150mA output short-circuit current, 7.5V/µs slew rate,
and 12MHz bandwidth. The rail-to-rail input and output
capability maximizes system flexibility.
The operational amplifier limits short-circuit current to
approximately ±150mA if the output is directly shorted to
SUPB or to GND. If the short-circuit condition persists,
the junction temperature of the IC rises until it reaches
the thermal shutdown threshold (+160°C typ). Once the
junction temperature reaches the thermal shutdown
threshold, an internal thermal sensor immediately sets
the thermal fault latch, shutting off all the IC’s outputs.
The device remains inactive until the input voltage is
cycled or SHDN is toggled.
The operational amplifier is typically used to drive the
LCD backplane (VCOM) or the gamma-correction
divider string. The LCD backplane consists of a distrib-
uted series capacitance and resistance, a load that can
be easily driven by the operational amplifier. However,
if the operational amplifier is used in an application with
a pure capacitive load, steps must be taken to ensure
stable operation.
As the operational amplifier’s capacitive load increases,
the amplifier’s bandwidth decreases and gain peaking
increases. A 5Ω to 50Ω small resistor placed between
OUTB and the capacitive load reduces peaking but also
reduces the gain. An alternative method of reducing
peaking is to place a series RC network (snubber) in par-
allel with the capacitive load. The RC network does not
continuously load the output or reduce the gain. Typical
values of the resistor are between 100Ω and 200Ω and
the typical value of the capacitor is 10pF.
The MAX8758’s high-voltage switch-control block (Figure
5) consists of two high-voltage, p-channel MOSFETs: Q1,
between SRC and GON and Q2, between GON and
DRN. The switch-control block is enabled when V
exceeds V
CTL and MODE. There are two different modes of opera-
tion (see the Typical Operating Characteristics section.)
Table 3. Frequency Selection
FREQ
GND
IN
LDO
/2 and then Q1 and Q2 are controlled by
High-Voltage Switch Control
Driving Pure Capacitive Load
SWITCHING FREQUENCY (kHz)
Short-Circuit Current Limit
Operational Amplifier
1200
600
DLP
13