BD91361MUV_14 Datasheet, PDF (14/27 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD91361MUV
Datasheet
6. Consideration on Permissible Dissipation and Heat Generation
Since this IC functions with high efficiency without significant heat generation in most applications, no special
consideration is needed on permissible dissipation or heat generation. In case of extreme conditions, (such as lower
input voltage, higher output voltage, heavier load, and/or higher temperature), the permissible dissipation and/or heat
generation must be carefully considered.
For dissipation, only conduction losses due to DC resistance of inductor and ON-Resistance of FET are considered.
This is because the conduction losses are the most significant among other dissipation mentioned above including gate
charge/discharge dissipation and switching dissipation.
4
①3.56W
3
②2.21W
2
1
③0.70W
④0.34W
0
0
25 50 75 100105 125 150
Ambient temperature:Ta^[°C]
Figure 30. Thermal Derating Curve
(VQFN020V4040)
① 4 layers (Copper foil area : 5505mm²)
copper foil in each layers.
θj-a=35.1°C/W
② 4 layers (1^st and 4^th copper foil area : 10.29m²)
(2^nd and 3^rd copper foil area: 5505m²)
θj-a=56.6°C/W
③ 1 layer (Copper foil area : 10.29m²)
θj-a=178.6°C/W
④ IC only.
θj-a=367.6°C/W
P  I_OUT ²  R_ON
R_ON  D  R_ONH  1  DR_ONL
Where:
D is the ON duty (=VOUT/VCC).
RONH is the ON-Resistance of Highside MOSFET.
RONL is the ON-Resistance of Lowside MOSFET.
IOUT is the Output current.
If VCC=3.3V, VOUT=1.8V, RONH=60mΩ, RONL=55mΩ IOUT=4A
D  V_OUT / V_CC  1.8 / 3.3  0.545
R_ON  0.545 0.06  1 0.545 0.55
 0.0327 0.0250
 0.0577 
P  4²  0.0577  0.2309 W 
Since RONH is greater than RONL in this IC, the dissipation increases as the ON duty time increases. Taking into
consideration the dissipation as shown above, thermal design must be carried out with allowable sufficient margin.
www.rohm.com
© 2012 ROHM Co., Ltd. All rights reserved.
TSZ22111ï½¥15ï½¥001
14/23
TSZ02201-0J3J0AJ00200-1-2
06.Oct.2014 Rev.002