MAX8710ETG+T Maxim Integrated Products, MAX8710ETG+T Datasheet - Page 17

• Manufacturer Part Number: MAX8710ETG+T
• Description: Charge Pumps Linear-Reg LCD Panel Power Supply
• Manufacturer: Maxim Integrated Products

Specifications of MAX8710ETG+T

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

in parallel with the reservoir capacitor C
voltage across C
V
voltage (V
to C
The positive charge-pump regulator’s startup can be
delayed by connecting an external capacitor from DLP
to GND. An internal constant current source begins
charging the DLP capacitor when SHDN is logic high
and REF reaches regulation. When the DLP voltage
exceeds V
is enabled. Each time it is enabled, the positive charge-
pump regulator goes through a soft-start routine by
ramping up its internal reference voltage from 0 to 2.5V
in 128 steps. The soft-start period is 2.73ms (typ), and
FBP fault detection is disabled during this period. The
soft-start feature effectively limits the inrush current dur-
ing startup. The MAX8710/MAX8711/MAX8712/
MAX8761 also monitor the FBP voltage for undervolt-
age conditions. If V
for approximately 44ms, the device latches off.
The negative charge-pump regulator is typically used to
generate the negative supply rail for the TFT LCD gate-
driver ICs. The output voltage is set with an external resis-
tive voltage-divider from its output to REF with the mid-
point connected to FBN. The number of charge-pump
stages and the setting of the feedback divider determine
the output of the negative charge-pump regulator. The
charge-pump driver includes a high-side p-channel
MOSFET (P2) and a low-side n-channel MOSFET (N2) to
control the power transfer as shown in Figure 5. The
MOSFETs switch a constant frequency of 1.5MHz.
During the first half-cycle, P2 turns on and allows
V
through diode D3. During the second half-cycle, P2 turns
off and N2 turns on, level shifting C
volts. This connects C
capacitor C
minus a diode drop is greater than the voltage across
C
diode D4 turns off. The amount of charge transferred to
the output is controlled by the on-resistance of N2, which
varies according to the output of the feedback error
amplifier. The error amplifier compares the feedback sig-
nal (FBN) with a 250mV internal reference and amplifies
the difference. If the feedback signal is above the refer-
ence, the error-amplifier output increases the supply volt-
age of N2’s gate driver, lowering the on-resistance.
Similarly, if the feedback signal is below the reference,
DIODE
INPUT
X(NEG)
OUT(POS)
) is smaller than the level-shifted flying-capacitor
, charge flows from C
to charge up the flying capacitor C
CX(POS)
REF
OUT(NEG)
Negative Charge-Pump Regulator
until diode D2 turns off.
/ 2, the positive charge-pump regulator
OUT(POS)
+ V
______________________________________________________________________________________
FBP
. If the voltage across C
INPUT
X(NEG)
is continuously below 2V (typ)
), charge flows from C
plus a diode drop (V
OUT(NEG)
in parallel with reservoir
X(NEG)
to C
OUT(POS)
X(NEG)
by V
OUT(NEG)
X(NEG)
. If the
POS
X(POS)
Low-Cost, Linear-Regulator
INPUT
until
LCD Panel Power Supplies
+
the error-amplifier output reduces the driver supply
voltage, increasing the on-resistance.
The negative charge-pump regulator is enabled when
SHDN is logic high and REF reaches regulation. Each
time it is enabled, the negative charge-pump regulator
goes through a soft-start routine by ramping down its
internal reference voltage from 5V to 250mV in 128
steps. The soft-start period is 2.73ms (typ), and FBN
fault detection is disabled during this period. The soft-
start feature effectively limits the inrush current during
startup. The MAX8710/MAX8711/MAX8712/MAX8761
also monitor the FBN voltage for undervoltage condi-
tions. If V
approximately 44ms, the device latches off.
The MAX8710/MAX8711/MAX8761s’ operational ampli-
fier features high output current (150mA), fast slew rate
(7.5V/µs), and wide bandwidth (12MHz). The opera-
tional amplifier is enabled when REF is in regulation
and SHDN is logic high. The output of the amplifier
(OUTB) is internally pulled to ground through a 1kΩ
resistor in shutdown.
The amplifier is typically used to drive the backplane
(VCOM) of TFT LCD panels. The LCD backplane
consists of a distributed series capacitance and resis-
tance, a load that can be easily driven by this opera-
tional 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 its gain peaking
increases. To ensure stable operation, a 5Ω to 50Ω
resistor can be placed between OUTB and the capaci-
tive load to reduce gain peaking.
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
trips the IC’s thermal-overload protection.
The reference output is nominally 5V and can source
up to 1mA (see the Typical Operating Characteristics).
Bypass REF with a 0.22µF ceramic capacitor connect-
ed between REF and GND. The reference remains
enabled in shutdown.
FBN
is continuously above 700mV (typ) for
(MAX8710/MAX8711/MAX8761)
Reference Voltage (REF)
Operational Amplifier
17