MIC26903 Datasheet, PDF (17/31 Pages) Micrel Semiconductor – 28V, 9A Hyper Light Load Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 4. MIC26903 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
MIC26903 to achieve high efficiency in light load
applications.
V_DD Regulator
The MIC26903 provides a 5V regulated output for input
voltage V_IN ranging from 5.5V to 28V. 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 MIC26903 implements an internal digital soft-start
by making the 0.8V reference voltage V_REF ramp from 0
to 100% in about 5ms 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.
MIC26903
Current Limit
The MIC26903 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 MIC26903 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the inductor current is
greater than 15A, then the MIC26903 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.
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.
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. MIC26903 Current Limit Foldback Characteristic
July 2011
17
M9999-071311-A