Rainbow Electronics MAX8712 - Manual (Page 16)

(V CX(POS) + V INPUT ), charge flows from C X(POS) to
C OUT(POS) until diode D2 turns off.
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
REF / 2, the positive charge-pump regulator
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 also
monitor the FBP voltage for undervoltage conditions. If
V
FBP is continuously below 2V (typ) for approximately
44ms, the device latches off.
Negative Charge-Pump Regulator
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 controller 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
INPUT to charge up the flying capacitor C X(NEG)
through diode D3. During the second half-cycle, P2
turns off and N2 turns on, level shifting C
X(NEG) by V IN-
PUT volts. This connects C X(NEG) in parallel with reser-
voir capacitor C
OUT(NEG)
. If the voltage across
C OUT(NEG) minus a diode drop is greater than the volt-
age across C X(NEG) , charge flows from C OUT(NEG) to
C X(NEG) until the 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 com-
pares the feedback signal (FBN) with a 250mV internal
reference and amplifies the difference. If the feedback
signal is above the reference, the error-amplifier output
increases the supply voltage of N2’s gate driver, lower-
ing the on-resistance. Similarly, if the feedback signal is
below the reference, 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 also monitor
the FBN voltage for undervoltage conditions. If V
FBN is
continuously above 700mV (typ) for approximately
44ms, the device latches off.
Operational Amplifier
(MAX8710/MAX8711)
The MAX8710/MAX8711s’ operational amplifier features
high output current (150mA), fast slew rate (7.5V/µs),
and wide bandwidth (12MHz). The operational 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.
Reference Voltage (REF)
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.
Power-Up Sequence and Shutdown Control
When the MAX8710/MAX8711/MAX8712 are powered
up, REF rises with the voltage on IN. After REF reaches
regulation and if SHDN is logic high, the linear regula-
tor, operational amplifier, and negative charge-pump
regulator are enabled and begin their respective soft-
start routines. After the soft-start routines are complet-
MAX8710/MAX8711/MAX8712
Low-Cost Linear-Regulator
LCD Panel Power Supplies
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