MIC24052 Datasheet, PDF (16/30 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

Micrel, Inc.
Figure 4. MIC24052 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
MIC24052 to achieve high efficiency in light load
applications.
VDD Regulator
The MIC24052 provides a 5V regulated output for input
voltage V_IN ranging from 5.5V to 19V. When V_IN < 5.5V,
V_DD 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 MIC24052 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.
MIC24052
Current Limit
The MIC24052 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 MIC24052 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the inductor current is
greater than 11A, then the MIC24052 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 Thresold
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. MIC24052 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.
December 2012
16
M9999-120312-A