MIC7401 Datasheet, PDF (32/68 Pages) Micrel Semiconductor – Configurable PMIC, Five-Channel Buck Regulator plus One-Boost with HyperLight Load®, I2C Control, and Enable

Micrel, Inc.
MIC7401
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 V_OUT4 trace. The inductor current, Trace I_L4, 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 in a MIC7401 is a
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:
P_{D_BUCK} ≈ V_IN × I_IN – V_OUT × I_OUT – P_{L_LOSS} Eq. 4
While the boost power dissipation is estimated by
Equation 5:
P_{D_BOOST} ≈ V_IN × I_IN – V_OUT x I_OUT – P_{L_LOSS} – V_f ×
I_OUT
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:
P_{D_TOTAL} ≈ SUM (P_{D_SWITCHERS})
Eq. 6
March 16, 2015
32
Revision 1.0