RT2910A Datasheet, PDF (19/20 Pages) Richtek Technology Corporation – High Efficiency Inverting PWM Converter with Surge Stopper

capacitor supplies the load current before the controller
can respond.
Therefore, the ESR will dominate the output voltage
SAG during load transient. The output voltage under-
shoot (VSAG) can be calculated by the following
equation :
VSAG = ΔILOAD * ESR
For a given output voltage sag specification, the ESR
value can be determined.
Another parameter that has influence on the output
voltage sag is the equivalent series inductance (ESL).
The rapid change in load current results in di/dt during
transient.
Therefore, the ESL contributes to part of the voltage sag.
Using a capacitor with low ESL can obtain better
transient performance. Generally, using several
capacitors connected in parallel can have better
transient performance than using a single capacitor for
the same total ESR.
Inductor selection
There are different ways to calculate the required
inductance. A good way to do this is to design the
inductor current ripple current ΔIL between 20%~30% of
the average inductor current IL. This will make the
regulator designed into a good load transient response
with an acceptable output ripple voltage.
Therefore, we suggest peak-to peak inductor current
ripple IL is designed as :
IL = 0.2 to 0.3 x IL
So required inductance :
L = (VIN x D) / (FS x IL)
Where D = VOUT / (VIN + VOUT)
Thermal Protection
The device implements an internal thermal shutdown
function when the junction temperature exceeds 150°C.
The thermal shutdown forces the device to stop loop
regulation and pull low POK. Once OTP release, the
RT2910A will soft-start again.
Thermal Considerations
The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
RT2910A
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 :
PD(MAX) = (TJ(MAX)  TA) / JA
where TJ(MAX) is the maximum junction temperature, TA
is the ambient temperature, and 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, JA, is highly package
dependent. For a WQFN-24L 5x5 package, the thermal
resistance, JA, is 28C/W on a standard JEDEC 51-7
high effective-thermal-conductivity four-layer test board.
The maximum power dissipation at TA = 25C can be
calculated as below :
PD(MAX) = (125C  25C) / (28C/W) = 3.57W for a
WQFN-24L 5x5 package.
The maximum power dissipation depends on the
operating ambient temperature for the fixed TJ(MAX) and
the thermal resistance, JA. The derating curves in
Figure 2 allows the designer to see the effect of rising
ambient temperature on the maximum power
dissipation.
4.0
Four-Layer PCB
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 8. Derating Curve of Maximum Power
Dissipation
Copyright © 2017 Richtek Technology Corporation. All rights reserved.
DS2910A-00 August 2017
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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