MB39C014 datasheet(16/32 Pages) CYPRESS

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Document Number: 002-08361 Rev. *C

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MB39C014

Furthermore, the continuity loss (PC) is divided roughly into the loss by internal SW FET ON-resistance and by external inductor series

resistance.

= IOUT2 × (RDC + D × RONP + (1 − D) × RONN)

: Switching ON-duty cycle (

= VOUT / VIN)

RONP

: Internal P-ch SW FET ON resistance

RONN

: Internal N-ch SW FET ON resistance

RDC

: External inductor series resistance

IOUT

: Load current

The above formula indicates that it is important to reduce RDC as much as possible to improve efficiency by selecting components.

10.4 Power Dissipation and Heat Considerations

The IC is so efficient that no consideration is required in most of the 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 :

= IOUT2 × (D × RONP + (1 − D) × RONN)

: Switching ON-duty cycle (

= VOUT / VIN)

RONP

: Internal P-ch SW FET ON resistance

RONN

: Internal N-ch SW FET ON resistance

IOUT

: 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 = 0.42 Ω and RONN = 0.36 Ω according to the graph “MOS FET ON

resistance vs. Operating ambient temperature”. The IC's internal loss P is 144 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 539 mW and the internal loss is smaller than the power dissipation.

10.5 Transient Response

Normally, IOUT is suddenly changed while VIN and VOUT are maintained constant, responsiveness including the response time and

overshoot/undershoot voltage is checked. As this IC has built-in Error Amp with an optimized design, it shows good transient response

characteristics. However, if ringing upon sudden change of the load is high due to the operating conditions, add capacitor C6 (e.g. 0.1

µF). (Since this capacitor C6 changes the start time, check the start waveform as well.) This action is not required for DAC input.

VREF

VREFIN

VREF

VREFIN

MB39C014

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