MIC2164 MICREL [Micrel Semiconductor], MIC2164 Datasheet - Page 13

• Manufacturer Part Number: MIC2164
• Description: Constant Frequency, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
With the varying duty cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small in MIC2164/-2/-3 converter.
Unlike the current-mode control, MIC2164/-2/-3 uses the
output voltage ripple, which is proportional to the
inductor current ripple if the ESR of the output capacitor
is large enough, to trigger an ON-time period. The
predetermined ON time makes MIC2164/-2/-3 control
loop has the advantage as the adaptive on-time mode
control. Therefore, the slope compensation, which is
necessary for the current-mode control, is not required in
the MIC2164/-2/-3 family.
The MIC2164/-2/-3 family has its own stability concern:
the FB voltage ripple should be in phase with the
inductor current ripple and large enough to be sensed by
the gm amplifier and the error comparator. The
recommended minimum FB voltage ripple is 20mV. If a
low ESR output capacitor is selected, the FB voltage
ripple may be too small to be sensed by the gm amplifier
and the error comparator. Also, the output voltage ripple
and the FB voltage ripple are not in phase with the
inductor current ripple if the ESR of the output capacitor
is very low. Therefore, the ripple injection is required for
a low ESR output capacitor. Please refer to “Ripple
Injection” subsection in “Application Information” for
more details about the ripple injection.
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.
MIC2164/-2/-3 implements an internal digital soft-start by
making the 0.8V reference voltage V
100% in about 6ms with a 9.7mV step. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V
September 2009
Figure 3. MIC2164/-2/-3 Load-Transient Response
REF
ramp. Once the soft-start ends, the related
REF
ramp from 0 to
13
circuitry is disabled to reduce the current consumption.
V
the soft-start function behavior correctly.
Current Limit
The MIC2164/-2/-3 uses the R
power MOSFET to sense over-current conditions. The
lower-side MOSFET is used because it displays much
lower parasitic oscillations during switching then the
high-side MOSFET. Using the low-side MOSFET R
as a current sense is an excellent method for circuit
protection. This method will avoid adding cost, board
space and power losses taken by discrete current sense
resistors.
In each switching cycle of the MIC2164/-2/-3 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
the MIC2164/-2/-3 turns off the high-side MOSFET and a
soft-start sequence is trigged. This mode of operation is
called the “hiccup mode” and its purpose is to protect the
down stream load in case of a hard short. The current
limit threshold V
to the FB voltage. Please refer to the “Typical
Characteristics” for the curve of V
circuit in Figure 4 illustrates the MIC2164/-2/-3 current
limiting circuit.
Using the typical V
value is roughly estimated as:
For designs where the current ripple is significant
compared to the load current I
operation, calculating the current limit I
into account that one is sensing the peak inductor
current and that there is a blanking delay of
approximately 150ns.
IN
CL
, which is 130mV typical at 0.8V feedback voltage,
should be powered up no earlier than V
Figure 4. MIC2164/-2/-3 Current Limiting Circuit
CL
has a fold back characteristics related
CL
I
CL
value of 130mV, the current limit
≈
R
130mV
DS(ON)
OUT
DS(ON)
CL
, or for low duty cycle
vs. FB voltage. The
M9999-090409-B
of the low-side
MIC2164/-2/-3
CL
HSD
should take
to make
CL
DS(ON)
after