RT8450BGCP Richtek USA Inc, RT8450BGCP Datasheet - Page 12

• 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

RT8450/B
Over Temperature Protection
The
function to
overheating. The OTP function will shut down switching
operation when the die junction temperature exceeds
150°C. The chip will automatically start to switch again
when the die junction temperature cools off.
Inductor Selection
Choose an inductor that can handle the necessary peak
current without saturating, and ensure that the inductor
has a low DCR (copper-wire resistance) to minimize I
power losses. A 4.7μH to 10μH inductor will meet the
demand for most of the
manufacturers specify the maximum current rating as the
current where the inductance falls to certain percentage
of its nominal value—typically 65%.
In Boost application where the transition between
discontinuous and continuous modes occurs, the value
of the required output inductor (L), can be approximated
by the following equation :
The Duty Cycle (D) could be calculated as follows :
Where V
The boost converter operates in discontinuous mode over
the entire input voltage range can have inductor value L1
less than the calculated value L by the formula above.
With an inductance value L2 greater than L, the converter
will operate in continuous mode at the minimum input
voltage and maybe operate in discontinuous mode at
higher voltages.
The inductor must be selected with a saturation current
rating greater than the peak current provided by the
following equation :
Where ç is the efficiency of the power converter.
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12
L =
D =
I
PEAK
RT8450/B has over temperature protection (OTP)
V
V
OUT
OUT
=
I
f = operating frequency.
V
OUT
V
2 x I
OUT
IN
OUT
V
OUT
prevent the excessive power dissipation from
x D x (1-D)
= minimum input voltage.
- V
= sum of current from all LED strings.
η x V
OUT
= maximum output voltage.
IN
x I
IN
x f
LED
2
+
RT8450/B
V
IN
2 x L
x D x T
applications. Inductor
2
R
Schottky Diode Selection
The Schottky diode,
fast switching speed, is necessary for the RT8450/B
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are the important
parameters of the Schottky diode that must be considered.
Choose a suitable Schottky diode whose reverse voltage
rating is greater than the maximum output voltage. The
diode’ s average current rating must exceed the average
output current. The diode conducts current only when the
power switch is turned off (typically less than 50% duty
cycle).
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most RT8450/B applications, a 4.7μF ceramic capacitor
is sufficient. A value higher or lower may be used
depending on the
the input current to the converter.
In Boost Application, the output capacitor is typically a
ceramic capacitor and is selected based on the output
voltage ripple requirements. The minimum value of the
output capacitor C
following equation :
Layout Consideration
PCB layout is very important to design power switching
converter circuits. Some recommended layout guidelines
are suggested as follows:
C
The power components L1, D1, C
Place L1 and D1 connected to SW pin as close as
OUT
placed as close to each other as possible to reduce the
ac current loop area. The PCB trace between power
components must be as short and wide as possible
due to large current flow through these traces during
operation.
possible. The trace should be as short and wide as
possible.
as close to VCC1 and VCC2 pin as possible.
The input capacitors C
=
I
LED
V
RIPPLE
x D x T
noise level from the input supply and
OUT
with low forward voltage drop and
is approximately given by the
VCC1
DS8450/B-02 November 2009
and C
VIN
VCC2
, and C
must be placed
OUT
must be