RT9921PQV 查看數據表（PDF） - Richtek Technology
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RT9921PQV Datasheet PDF : 15 Pages
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Preliminary
RT9921
Application Information
The RT9921 contains a high performance boost regulator
to generate voltage for output voltage, gate-on driver and
gate-off driver. It also includes of a high-current rail-to rail
operation amplifier, a gate pulse modulator (GPM), a
programmable timing control voltage detector, and a low
dropout linear regulator. The following content contains
the detailed description and the information of the
component selection.
Boost Regulator
The boost regulator is a high efficiency current-mode PWM
architecture with 640k / 1.2MHz operation frequency. It
performs fast transient responses to generate source driver
supplies for TFT LCD display. The high operation frequency
allows smaller components to minimize the thickness of
LCD panel. To regulate the output voltage is to set resistive
voltage-divider sensing at FB pin. The error amplifier varies
the COMP voltage by sensing the FB pin to regulate the
output voltage. For better stability, the slope compensation
signal that combined with the current-sense signal will
be compared with the COMP voltage to determine the
current trip point and duty cycle.
Soft-Start
The RT9921 provides soft-start function to minimize the
inrush current. When power on, an internal constant current
charges an external capacitor. The rising voltage rate on
COMP pin will be limited during the charging period and
the inductor peak current will also be limited at the same
time. In the meanwhile, the frequency increases slowly at
the beginning. When power off, the external capacitor will
be discharged for next soft start time.
The soft-start function is implemented by the external
capacitor with a 4μA constant current charging to the
softstart capacitor. Therefore, the capacitor should be large
enough for output voltage regulation. Typical value for
softstart capacitor range is 27nF. The available soft start
capacitor range is from 10nF to 100nF.
Output Voltage
The regulated output voltage is calculated by the following
formula :
VOUT
=
VFB × ⎛⎜⎝1+
R1
R2
⎞⎟⎠,
where
VFB
=
1.24V
(typ.)
The recommended value for R2 should be up to 100kΩ
without some sacrificing. To place the resistor-divider as
close as possible to the chip can reduce noise sensitivity.
Loop Compensation
The voltage feedback loop can be compensated with an
external compensation network consisted of R3, C3 (As
Figure 1). Choosing R3 to set high frequency integrator
gain for fast transient response and C3 to set the integrator
zero to maintain loop stability. For typical application
VIN = 3.3V , VOUT = 8.5V , COUT = 4.7μF x 3 , L = 4.7μH,
the recommened value for compensation is as below :
R3 = 56kΩ, C3 = 1nF.
Over Current Protection
The RT9921 main boost converter has the function of over-
current from protection to limit peak inductor current. It
prevents large current damaging the inductor and diode.
During the ON-time, once the inductor current exceeds
the current limit, the internal LX switch turns off
immediately and shortens the duty cycle. Therefore, the
output voltage drops if the over-current condition occurs.
Actual current limit is always larger than the nominal value
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because of the internal circuit delay. Current limit is also
affected by the input voltage, duty cycle, and inductor
value.
Over Temperature Protection
The RT9921 main boost converter has thermal protection
function to prevent the excessive power dissipation from
overheating. When the junction temperature exceeds
170°C, it will shut down the device. Once the device cools
down by approximately 20°C, it will start to operate
normally. For continuous operation, do not operate over
the maximum junction temperature rating around 150°C.
Under Voltage Protection
If the boost regulator feedback voltage is under 90% of
the nominal value, the RT9921 activates an internal timer.
If the fault condition continues for 160ms, the PWM will
latch off and won't restart until VIN power is cycled. The
under-voltage protection is disabled during the soft-start
time.
DS9921-00 November 2007
www.richtek.com
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