MIC26901 MICREL [Micrel Semiconductor], MIC26901 Datasheet - Page 16

• Manufacturer Part Number: MIC26901
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Figure 3 shows the operation of the MIC26901 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 MIC26901 converter.
Unlike true current-mode control, the MIC26901 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 MIC26901 control loop has the advantage
of eliminating the need for slope compensation.
July 2011
Figure 3. MIC26901 Load Transient Response
REF
Figure 2. MIC26901 Control Loop Timing
. This will cause the error comparator to trigger
OFF(min)
is generated to charge C
REF
FB
. Then, the
to be less
BST
16
In order to meet the stability requirements, the
MIC26901 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,
then 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.
V
The MIC26901 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 MIC26901 implements an internal digital soft-start
by making the 0.8V reference voltage V
to 100% in about 6ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V
related
consumption. V
or after V
Current Limit
The MIC26901 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 MIC26901 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor current
is greater than 15A, then the MIC26901 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 foldback characteristic related to the feedback
voltage as shown in Figure 4.
DD
DD
Regulator
should be tied to PVIN pins to bypass the internal
recommended
FB
IN
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
DD
m
m
must be powered up at the same time
amplifier and the error comparator.
amplifier and the error comparator.
is
feedback
disabled
DS(ON)
of the internal low-side
to
voltage
M9999-071311-A
reduce
REF
ramp from 0
MIC26901
ripple
IN
< 5.5V,
current
is