MIC2174 Datasheet, PDF (11/24 Pages) Micrel Semiconductor – 300kHz, Synchronous Buck Controller 300kHz, Synchronous Buck Controller

Micrel, Inc.
MIC2174
comparator to trigger ON-time period. At the end of the
ON-time period, a minimum OFF time T_OFF(min) is
generated to charge BST since the FB voltage is still
below the V_REF. Then, the next ON-time period is
triggered due to the low FB voltage. Therefore, the
switching frequency changes during the load transient.
With the varying duty cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small in MIC2174 converter.
Figure 3. MIC2174 Load-Transient Response
Unlike the current-mode control, MIC2174 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 MIC2174 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 MIC2174.
The MIC2174 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.
MIC2174 implements an internal digital soft-start by
making the 0.8V reference voltage V_REF ramp from 0 to
100% in about 6ms with a 9.7mV step. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V_REF ramp. Once the soft-start ends, the related
circuitry is disabled to reduce the current consumption.
V_IN should be powered up no earlier than V_HSD to make
the soft-start function behavior correctly.
Current Limit
The MIC2174 uses the R_DS(ON) of the low-side 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_DS(ON) 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 MIC2174 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_CL after
a blanking time of 150ns. If the sensed voltage is over
V_CL, which is 130mV typical at 0.8V feedback voltage,
the MIC2174 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_CL has a fold back characteristics related
to the FB voltage. Please refer to the “Typical
Characteristics” for the curve of V_CL vs. FB voltage. The
circuit in Figure 4 illustrates the MIC2174 current limiting
circuit.
Figure 4. MIC2174 Current Limiting Circuit
Using the typical V_CL value of 130mV, the current limit
value is roughly estimated as:
I_CL
≈
130mV
R_DS(ON)
For designs where the current ripple is significant
compared to the load current I_OUT, or for low duty cycle
operation, calculating the current limit I_CL should take
into account that one is sensing the peak inductor
September 2009
11
M9999-090409-B