MIC7401 datasheet(32/68 Pages) MICREL

32 / 68 page

Micrel, Inc.

MIC7401

March 16, 2015

Revision 1.0

Figure 14. STBY-to-NORMAL Transition (DEFAULT)

Safe Start-Up into a Pre-Biased Output

The MIC7401 is designed for safe start-up into a pre-

biased output. This prevents large negative inductor

currents which can cause the output voltage to dip and

excessive output voltage oscillations. A zero crossing

comparator is used to detect a negative inductor current.

If a negative inductor current is detected, the low-side

synchronous MOSFET functions as a diode and is

immediately turned off.

Figure 15 shows a 1V output pre-bias at 0.5V at start-up,

see VOUT4 trace. The inductor current, Trace IL4, is not

allowed to go negative by more than 0.5A before the low-

side switch is turned off. This feature prevents high

negative inductor current flow in a pre-bias condition

which can damage the IC.

Figure 15. Pre-Biased Output Voltage

Buck Regulator Power Dissipation

The

total

power

dissipation

MIC7401

combination of the five buck regulators and the boost

dissipation. The buck regulators (OUT1 to OUT5)

dissipation is approximately the switcher’s input power

minus the switcher’s output power and minus the power

loss in the inductor:

PD_BUCK

≈ V

IN × IIN – VOUT × IOUT – PL_LOSS

Eq. 4

While the boost power dissipation is estimated by

Equation 5:

PD_BOOST

≈ V

IN × IIN – VOUT x IOUT – PL_LOSS – Vf ×

IOUT

Eq. 5

Although the maximum output current for a single buck

regulator can be as much as 3A, the MIC7401 will

thermal limit and will not support this high output current

on all outputs at the same time.

Total Power Dissipation

The total power dissipation in the MIC7401 package is

equal to the sum of the power loss of each regulator:

PD_TOTAL

≈ SUM (P

D_SWITCHERS)

Eq. 6

MIC7401 Datasheet(PDF) 32 Page - Micrel Semiconductor

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