MIC26950 MICREL, MIC26950 Datasheet - Page 14

• Manufacturer Part Number: MIC26950
• Description: 12A Hyper Speed Control Synchronous DC-DC Buck Regulator
• Manufacturer: MICREL

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Micrel, Inc.
Figure 3 shows the operation of the MIC26950 during a
load transient. The output voltage drops due to the
sudden load increase, which causes the V
than V
an ON-time period. At the end of the ON-time period, a
minimum OFF-time t
since the feedback voltage is still below V
next ON-time period is triggered due to the low feedback
voltage. Therefore, the switching frequency changes
during the load transient, but returns to the nominal fixed
frequency once the output has stabilized at the new load
current level. With the varying duty cycle and switching
frequency, the output recovery time is fast and the
output voltage deviation is small in MIC26950 converter.
Unlike true current-mode control, the MIC26950 uses the
output voltage ripple to trigger an ON-time period. The
output voltage ripple is proportional to the inductor
current ripple if the ESR of the output capacitor is large
enough. The MIC26950 control loop has the advantage
of eliminating the need for slope compensation.
In order to meet the stability requirements, the
MIC26950 feedback voltage ripple should be in phase
with the inductor current ripple and large enough to be
sensed by the g
The
20mV~100mV. If a low ESR output capacitor is selected,
the feedback voltage ripple may be too small to be
sensed by the g
Also, the output voltage ripple and the feedback voltage
ripple are not necessarily in phase with the inductor
current ripple if the ESR of the output capacitor is very
low. In these cases, ripple injection is required to ensure
proper operation. Please refer to “Ripple Injection”
subsection in “Application Information” for more details
about the ripple injection technique.
September 2010
Figure 3. MIC26950 Load Transient Response
REF
recommended
. This will cause the error comparator to trigger
m
m
amplifier and the error comparator.
amplifier and the error comparator.
OFF(min)
feedback
is generated to charge C
voltage
REF
FB
. Then, the
to be less
ripple
BST
is
14
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 MIC26950 implements an internal digital soft-start
by making the 0.8V reference voltage V
to 100% in about 6ms with a 9.7mV step. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V
related
consumption. V
or after V
correctly.
Current Limit
The MIC26950 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 MIC26950 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the inductor current is
greater than 27A, then the MIC26950 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 current limit threshold has a fold back
characteristic related to the feedback voltage. As shown
in Figure 4.
Figure 4. MIC26950 Current Limiting Circuit
FB
circuitry
30.0
25.0
20.0
15.0
10.0
5.0
0.0
IN
ramp. Once the soft-start cycle ends, the
0.0
to make the soft-start function behavior
DD
must be powered up at the same time
0.2
is
Current Limit Threshold
vs. Feedback Voltage
FEEDBACK VOLTAGE (V)
disabled
0.4
DS(ON)
0.6
of the internal low-side
to
0.8
M9999-091710-C
reduce
REF
ramp from 0
1.0
MIC26950
current