VLF4012AT-2R2M datasheet(17/43 Pages) CYPRESS

17 / 43 page

MB39C015

Document Number: 002-08364 Rev. *A

Page 17 of 43

The IC's control circuit loss (PCONT) is extremely small, less than 100 mW (with no load).

As the IC contains FETs which can switch faster with less power, the continuity loss (PC) is more predominant as the loss during heavy-

load operation than the control circuit loss (PCONT) and switching loss (PSW) .

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

resistance.

PC = 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 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)

: 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  0.36 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

 70 °C is 300 mW and the internal loss is smaller than the power dissipation.

VLF4012AT-2R2M Datasheet(PDF) 17 Page - Cypress Semiconductor