RT8453BGSP Richtek USA Inc, RT8453BGSP Datasheet - Page 9

• Manufacturer Part Number: RT8453BGSP
• Description: IC LED DVR BUCK 40V 1.5A 8SOP
• Manufacturer: Richtek USA Inc

Specifications of RT8453BGSP

• Constant Current: Yes
• Internal Driver: Yes
• Type - Primary: Backlight, General Purpose
• Mounting Type: Surface Mount
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Frequency: 400kHz ~ 600kHz
• Voltage - Supply: 4.5 V ~ 40 V
• Package / Case: 8-SOIC (0.154", 3.90mm Width) Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Operating Temperature: -
• Other names: 1028-1004-2

The ripple current ΔI
Where,
V
V
f = switching frequency.
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for RT8453B
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are important parameters
of the Schottky diode that must be considered. 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 RT8453B applications, a 4.7μF ceramic capacitor is
sufficient. A value higher or lower may be used depending
on the noise level from the input supply and the input current
to the converter.
In buck application, the output capacitor is typically
ceramic and selection is mainly based on the output voltage
ripple requirements. The output ripple, ΔV
by the following equation :
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of IC
package, PCB layout, rate of surrounding airflow and
temperature difference between junction to ambient. The
maximum power dissipation can be calculated by following
the formula :
P
Where T
DS8453B-00 August 2010
Δ =
Δ
OUT
IN =
D(MAX)
I
V
L
OUT
input voltage.
= output voltage.
⎡
⎢
⎣
V
= (T
≤ Δ ×
f L
J(MAX)
OUT
×
I
J(MAX)
L
⎤ ⎡
⎥ ⎢
⎦ ⎣
is the maximum junction temperature, T
×
⎡
⎢
⎣
ESR
1
− T
−
L
V
A
+
can be calculated :
V
) / θ
OUT
IN
8 f C
× ×
JA
⎤
⎥
⎦
1
OUT
⎤
⎥
⎦
OUT
, is determined
A
is
the ambient temperature and θ
thermal resistance.
For recommended operating conditions specification of
the RT8453B, the maximum junction temperature is 125°C
and T
thermal resistance θ
(exposed pad) packages, the thermal resistance θ
75° C/W on the standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at T
can be calculated by following formula :
P
SOP-8 (exposed pad)
The maximum power dissipation depends on operating
ambient temperature for fixed T
resistance θ
the designer to see the effect of rising ambient temperature
on the maximum power dissipation allowed.
Layout Considerations
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guide lines are as follows :
D(MAX)
The power components L1, D1 and C1 must be 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.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Figure 3. Derating Curve for RT8453B Package
A
is the ambient temperature.
= (125°C − 25°C) / (75°C/W) = 1.333W for
0
JA
. For RT8453B packages, the Figure 3
25
Ambient Temperature (°C)
JA
is layout dependent. For SOP-8
50
JA
is the junction to ambient
The junction to ambient
75
RT8453B
J(MAX)
Four-Layer PCB
www.richtek.com
and thermal
100
A
= 25°C
allows
JA
125
is