MIC2176-1 MICREL [Micrel Semiconductor], MIC2176-1 Datasheet - Page 15

• Manufacturer Part Number: MIC2176-1
• Description: Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Figure 3 shows the operation of the MIC2176 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 MIC2176 converter.
Unlike true current-mode control, the MIC2176 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 MIC2176 control loop has the advantage of
eliminating the need for slope compensation.
In order to meet the stability requirements, the MIC2176
feedback voltage ripple should be in phase with the
inductor current ripple and large enough to be sensed by
the g
recommended feedback voltage ripple is 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
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.
November 2010
REF
m
Figure 3. MIC2176 Load Transient Response
m
. This will cause the error comparator to trigger
amplifier and the error comparator. The
amplifier and the error comparator. Also, the
OFF(min)
is generated to charge C
REF
FB
. Then, the
to be less
BST
15
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 MIC2176 implements an internal digital soft-start by
making the 0.8V reference voltage V
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 MIC2176 uses the R
MOSFET to sense over-current conditions. This method
will avoid adding cost, board space and power losses
taken by discrete current sense resistors. 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 MIC2176 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. The sensed voltage is
compared with a current-limit threshold voltage V
a blanking time of 150ns. If the sensed voltage is over
V
MIC2176 turns off the high-side and low-side MOSFETs
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 V
related to the FB voltage. Please refer to the “Typical
Characteristics” for the curve of current limit threshold vs.
FB voltage percentage. The circuit in Figure 4 illustrates
the MIC2176 current limiting circuit.
CL
, which is 133mV typical at 0.8V V
FB
Figure 4. MIC2176 Current Limiting Circuit
IN
circuitry
ramp. Once the soft-start cycle ends, the
to make the soft-start function correctly.
DD
must be powered up at the same time
is
disabled
CL
DS(ON)
has a foldback characteristic
of the low-side power
to
REF
M9999-111710-A
reduce
ramp from 0 to
FB
, then the
MIC2176
CL
current
after