MIC261203 MICREL [Micrel Semiconductor], MIC261203 Datasheet - Page 17

• Manufacturer Part Number: MIC261203
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
During
comparator and the current limit comparator are turned
off. The bias current of most circuits are reduced. As a
result,
discontinuous mode is only about 450 μ A, allowing the
MIC261203 to achieve high efficiency in light load
applications.
V
The MIC261203 provides a 5V regulated output for input
voltage V
V
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 MIC261203 implements an internal digital soft-start
by making the 0.8V reference voltage V
to 100% in about 5ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V
related
consumption. V
or after V
July 2011
DD
DD
Regulator
should be tied to PVIN pins to bypass the internal
FB
Figure 4. MIC261203 Control Loop Timing
the
IN
discontinuous
IN
circuitry
ramp. Once the soft-start cycle ends, the
to make the soft-start function correctly.
ranging from 5.5V to 28V. When V
total
DD
(Discontinuous Mode)
must be powered up at the same time
is
power
disabled
mode,
supply
the
to
reduce
REF
current
zero
ramp from 0
IN
crossing
< 5.5V,
current
during
17
Current Limit
The MIC261203 uses the R
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 MIC261203 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the inductor current is
greater than 26A, then the MIC261203 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.
Figure 5. MIC261203 Current-Limit
30
25
20
15
10
5
0
0.0
Foldback Characteristic
0.2
Current Limit Threshold
FEEDBACK VOLTAGE (V)
vs. Feedback Voltage
0.4
DS(ON)
0.6
of the internal low-side
0.8
M9999-071311-A
1.0
MIC261203