MB39C007 Datasheet, PDF (17/52 Pages) Fujitsu Component Limited. – 2 ch DC/DC Converter IC Built-in Switching FET & voltage detection function, PFM/PWM Synchronous Rectification, and Down Conversion Support

MB39C007
[4] Power dissipation and heat considerations
The IC is so efficient that no consideration is required in most cases. However, if the IC is used at a low power
supply voltage, heavy load, high output voltage, or high temperature, it requires further consideration for higher
efficiency.
The internal loss (P) is roughly obtained from the following formula :
P = IOUT2 × (D × RONP + (1 − D) × RONN)
D
RONP
RONN
IOUT
: Switching ON-duty cycle ( = VOUT / VIN)
: Internal P-ch SW FET ON resistance
: Internal N-ch SW FET ON resistance
: Output current
The loss expressed by the above formula is mainly continuity loss. The internal loss includes the switching loss
and the control circuit loss as well but they are so small compared to the continuity loss they can be ignored.
In this IC with RONP greater than RONN, the larger the on-duty cycle, the greater the loss.
When assuming VIN = 3.7 V, Ta = + 70 °C, for example, RONP = 0.36 Ω and RONN = 0.30 Ω according to the
graph “MOS FET ON resistance vs. Operating ambient temperature”. The IC's internal loss P is 123 mW at
VOUT = 2.5 V and IOUT = 0.6 A. According to the graph “Power dissipation vs. Operating ambient temperature”,
the power dissipation at an operating ambient temperature Ta of + 70 °C is 300 mW and the internal loss is
smaller than the power dissipation.
DS04-27246-2E
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