RT8450BGCP Richtek USA Inc, RT8450BGCP Datasheet - Page 11

• Manufacturer Part Number: RT8450BGCP
• Description: IC LED DRIVER PWM 16TSSOP
• Manufacturer: Richtek USA Inc

Specifications of RT8450BGCP

• Internal Driver: Yes
• Type - Primary: Backlight
• Mounting Type: Surface Mount
• Topology: PWM, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Frequency: 400kHz ~ 600kHz
• Voltage - Supply: 4.5 V ~ 40 V
• Package / Case: 16-TSSOP (0.173", 4.40mm Width) Exposed pad
• Operating Temperature: -40°C ~ 85°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: 1028-1003-2

Application Information
The
buck, buck -boost and boost converter applications. This
device uses a fixed frequency, current mode control
scheme to provide excellent line and load regulation. The
control loop has a current sense amplifier to sense the
voltage between the ISP and ISN pins and provides an
output voltage at the VC pin. A PWM comparator then
turns off the internal power switch when the sensed power
switch current exceeds the compensated VC pin voltage.
The power switch will not reset by the oscillator clock in
each cycle. If the comparator does not turn off the switch
in a cycle, the power switch is on for more than a full
switching period until the comparator is tripped. In this
manner, the programmed voltage across the sense
resistor is regulated by the control loop.
The current through the sense resistor is set by the
programmed voltage and the sense resistance. The voltage
across the sense resistor can be programmed by either
the analog or PWM signals at the ACTL pin, or the PWM
signal at the DCTL pin.
The protection schemes in the RT8450/B include over-
temperature, input voltage under-voltage, output voltage
over-voltage, and switch current-limit.
Loop Compensation
The RT8450/B has an external compensation pin (VC)
allowing the loop response optimized for specific
application. An external resistor in series with a capacitor
is connected from the VC pin to GND to provide a pole
and a zero for proper loop compensation. The
recommended compensation resistance and capacitance
for the RT8450/B are 10k and 3.3nF.
Soft Start
The soft-start of the RT8450/B can be achieved by
connecting a capacitor from the SS pin to GND. The built-
in soft-start circuit reduces the start-up current spike and
output voltage overshoot. The soft-start time is determined
by the external capacitor charged by an internal 6μA
constant charging current. The SS pin directly limits the
rate of voltage rise on the VC pin, which in turn limits the
peak switch current. The value of the soft-start capacitor
is user-defined to satisfy the designer’ s
DS8450/B-02 November 2009
RT8450/B
is specifically designed to be operated in
requirement.
LED current Setting
The LED current could be calculated by the following
equation :
Where V(ISP-ISN) is the voltage between ISP and ISN
(190mV typ. if ACTL or DCTL dimming is not applied) and
the R2 is the resister between ISP and ISN.
Brightness / Dimming Control
The RT8450/B
control. Analog dimming is linearly controlled by an
external voltage (0.3V < V
output clamping amplifier and a resistor, PWM dimming
signal fed at DCTL pin can be easily low-pass filtered to
an analog dimming signal with one external capacitor from
ACTL pin to GND for noise-free PWM dimming. A very
high contrast ratio true digital PWM dimming can be
achieved by driving ACTL pin with a PWM signal from
100Hz to 10kHz.
Output Over Voltage Setting
The
(OVP) function. When the voltage at OVP pin exceeds a
threshold of approximately1.2V, the power switch is turned
off. The power switch can be turned on again once the
voltage at OVP pin drops below 1.2V.
For the Boost Application, the output voltage could be
clamped at a certain voltage level. The OVP voltage can
be set by the following equation :
Where R3 and R4 are the voltage divider from V
GND with the divider center node connected to OVP pin.
Current-Limit Protection
The
internal over current protection feature. In normal operation,
the power switch is turned off when the switch current
hits the loop-set value. The over current protection function
will turn off the power switch independent of the loop control
when the peak switch current reaches around 1.5A.
I
V
LED,MAX
OUT,OVP
RT8450/B
RT8450/B
=
= 1.2 x (1 +
V(ISP-ISN)
features both analog and digital dimming
is equipped with over voltage protection
can limit the peak switch current by the
R2
R3
R4
ACTL
)
< 1.2V). With an on-chip
RT8450/B
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OUT
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to