Rainbow Electronics MAX17117 - Manual (Page 15)

Internal-Switch Boost Regulator with Integrated
7-Channel Scan Driver, Op Amp, and LDO
MAX17117
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Overvoltage Protection
The MAX17117 monitors OPAS for an overvoltage con-
dition. If the OPAS voltage is above 17V (typ), the
MAX17117 disables the gate driver of the step-up regula-
tor and prevents the internal MOSFET from switching. The
OPAS overvoltage condition does not set the fault latch.
Overcurrent Protection
The step-up regulator features an adjustable cycle-
by-cycle current limit. The inductor current is sensed
through the LX switch during the LX switch on-time. If
the peak inductor current rises above the current-limit
threshold set by R
ENA , the LX switch immediately turns
off until the next switching cycle, effectively limiting the
peak-inductor current each cycle.
Soft-Start
The soft-start feature effectively limits the inrush current
at startup by slowly raising the regulation voltage of the
step-up converter’s feedback pin (V
FB
) at a rate deter-
mined by the selection of the soft-start capacitor (C SS ).
At startup, once ENA is pulled high through R ENA , an
internal 4FA (typ) current source begins to charge the
soft-start capacitor (C
SS ), slowly bringing up the volt-
age at the soft-start pin (V SS ). V FB follows V SS for V SS <
1.24V. Once V SS exceeds 1.24V, V FB remains at 1.24V,
allowing V MAIN to reach its full regulation voltage.
Fault Protection
During steady-state operation, the MAX17117 monitors
the FB voltage. If the FB voltage falls below 1.1V (typ),
the MAX17117 activates an internal fault timer. If there is
a continuous fault more than 160ms (typ), the MAX17117
sets the fault latch, turning off all outputs except LDOO.
Once the fault condition is removed, cycle the input volt-
age to clear the fault latch and reactivate the device. The
fault-detection circuit is disabled during the soft-start time.
Operational Amplifier
The MAX17117 has an operational amplifier that is
typically used to drive the LCD backplane (VCOM) or
the gamma-correction-divider string. The operational
amplifier features Q200mA (typ) output short-circuit cur-
rent, 40V/Fs (typ) slew rate, and 16MHz (typ) bandwidth.
While the op amp is a rail-to-rail input and output design,
its accuracy is significantly degraded for input voltages
within 1V of its supply rails (OPAS and AGND).
Short-Circuit Current Limit
The operational amplifier limits short-circuit current to
approximately Q200mA (typ) if the output is directly
shorted to OPAS or to AGND. If the short-circuit condi-
tion persists, the junction temperature of the IC rises until
it reaches the thermal-shutdown threshold (+170NC typ).
Once the junction temperature reaches the thermal-shut-
down threshold, an internal thermal sensor immediately
shuts down all outputs until the input voltage is cycled
off, then on again.
Driving Pure Capacitive Loads
The operational amplifier is typically used to drive the
LCD backplane (VOUT) or the gamma-correction-divider
string. The LCD backplane consists of a distributed
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 5I to 50I small resistor placed
between VOUT and the capacitive load reduces peak-
ing, but also reduces the gain. An alternative method
of reducing peaking is to place a series RC network
(snubber) in parallel with the capacitive load. The RC
network does not continuously load the output or reduce
the gain. Typical values of the resistor are between 100I
and 200I and the typical value of the capacitor is 10pF.
High-Voltage Scan Driver
The high-voltage, level-shifting scan driver with gate-
shading control is designed to drive the TFT panel
gate drivers. Its seven outputs swing 40V (maximum)
between +35V (maximum) and -15V (minimum) and can
swiftly drive capacitive loads. The driver outputs (STH,
CKH_) swing between their power-supply rails (GHON
and VGL), according to the input logic levels on their
corresponding inputs (ST, CK_) except during a gate-
shading period. During a gate-shading period, a CKH_
output driver becomes high impedance and an internal
switch connected between the CKH_ output’s capaci-
tive load and either RO or RE closes (S1–S6) whenever
the state of its corresponding CK_ input is logic-low.
This allows part of an output’s GHON-to-VGL transition
to be completed by partially discharging its capacitive
load through an external resistor attached to either RO
or RE for a duration set by the gate-shading period. See
Figure 4.