MIC24054 Datasheet, PDF (16/30 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

Micrel, Inc.
Figure 4. MIC24054 Control Loop Timing
(Discontinuous Mode)
During discontinuous mode, the zero crossing
comparator and the current limit comparator are turned
off. The bias current of most circuits are reduced. As a
result, the total power supply current during
discontinuous mode is only about 450μA, allowing the
MIC24054 to achieve high efficiency in light load
applications.
VDD Regulator
The MIC24054 provides a 5V regulated output for input
voltage VIN ranging from 5.5V to 19V. When VIN < 5.5V,
VDD should be tied to PVIN pins to bypass the internal
linear regulator.
Soft-Start
Soft-start reduces the power supply input surge current
at startup by controlling the output voltage rise time. The
input surge appears while the output capacitor is
charged up. A slower output rise time will draw a lower
input surge current.
The MIC24054 implements an internal digital soft-start
by making the 0.8V reference voltage V_REF ramp from 0
to 100% in about 3ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V_FB ramp. Once the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption. V_DD must be powered up at the same time
or after V_IN to make the soft-start function correctly.
MIC24054
Current Limit
The MIC24054 uses the R_DS(ON) of the internal low-side
power MOSFET to sense over-current conditions. This
method will avoid adding cost, board space and power
losses taken by a discrete current sense resistor. The
low-side MOSFET is used because it displays much
lower parasitic oscillations during switching than the
high-side MOSFET.
In each switching cycle of the MIC24054 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the inductor current is
greater than 14A, then the MIC24054 turns off the high-
side MOSFET and a soft-start sequence is triggered.
This mode of operation is called “hiccup mode” and its
purpose is to protect the downstream load in case of a
hard short. The load current-limit threshold has a fold
back characteristic related to the feedback voltage as
shown in Figure 5.
Current Limit Threshold
vs. Feedback Voltage
20
16
12
8
4
0
0.0
0.2
0.4
0.6
0.8
1.0
FEEDBACK VOLTAGE (V)
Figure 5. MIC24054 Current-Limit Foldback Characteristic
Power-Good (PG)
The Power Good (PG) pin is an open drain output which
indicates logic high when the output is nominally 92% of
its steady state voltage. A pull-up resistor of more than
10kΩ should be connected from PG to VDD.
October 2012
16
M9999-102512-A