DS8409 Datasheet, PDF (9/11 Pages) Richtek Technology Corporation – Green Mode Buck Converter

RT8409
f_SW is switching frequency from 30kHz to 55kHz.
t_OFF is constant off time (25μs, Typ.).
Forward Diode Selection
When the power switch turns off, the path for the current
is through the diode connected between the switch output
and ground. This forward biased diode must have minimum
voltage drop and recovery time. The reverse voltage rating
of the diode should be greater than the maximum input
voltage and the current rating should be greater than the
maximum load current.
The peak voltage stress of diode is :
    ^{VD = 1.2 2  VACMAX = 1.2 2  264 = 448V}
The input source is universal (V_IN = 90V to 264V), VD will
reach 448V.
Bootstrap Diode Selection
The bootstrap diode is connected the switch output to
provide supply voltage for VCC, and output voltage for FB
divided resistors. The reverse voltage rating of the diode
should be greater than maximum input voltage. A fast
diode can be used, such as ES1J or FR107.
Sense Resistor Selection
The resistor, R5, between the Source of the external N-
MOSFET and SGND should be selected to provide
adequate switch current to drive the application without
exceeding the current limit threshold set by the SENSE
pin sense threshold of the RT8409. The Sense resistor
value can be calculated according to the formula below :
R2  ^VCLT
I_OCP
where V_CTL is the current limit threshold (0.97V, min.).
I_OCP is about 1.2 to 1.5 times of the peak inductor current.
Thermal Protection (OTP)
A thermal protection feature is included to protect the
RT8409 from excessive heat damage. When the junction
temperature exceeds a threshold of 150°C, the thermal
protection OTP will be triggered and the RT8409 will be
turned off.
Thermal Considerations
The junction temperature should never exceed the
absolute maximum junction temperature T_J(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
package, the PCB layout, the rate of surrounding airflow,
and the difference between the junction and ambient
temperatures. The maximum power dissipation can be
calculated using the following formula :
P_D(MAX) = (T_J(MAX) − T_A) / θ_JA
where T_J(MAX) is the maximum junction temperature, T_A is
the ambient temperature, and θ_JA is the junction-to-ambient
thermal resistance.
For continuous operation, the maximum operating junction
temperature indicated under Recommended Operating
Conditions is 125°C. The junction-to-ambient thermal
resistance, θ_JA, is highly package dependent. For a
SOP-8 package, the thermal resistance, θ_JA, is
206.9°C/W on a standard JEDEC low effective-thermal-
conductivity two-layer test board. The maximum power
dissipation at T_A = 25°C can be calculated as below :
P_D(MAX) = (125°C − 25°C) / (206.9°C/W) = 0.48W for a
SOP-8 package.
The maximum power dissipation depends on the operating
ambient temperature for the fixed T_J(MAX) and the thermal
resistance, θ_JA. The derating curves in Figure 1 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS8409-00 September 2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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