MIC2165_1011 Datasheet, PDF (13/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
parasitic oscillations during switching than the high-side
MOSFET.
In each switching cycle of the MIC2165 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 133mV typical at 0.8V V_FB, then the MIC2165 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_CL
has a foldback characteristic 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 5 illustrates the MIC2165 current limiting circuit.
Figure 4. MIC2165 Control Loop Timing
(Discontinuous Mode)
An external Schottky diode D2 is recommended in
parallel with the low-side MOSFET for high efficiency
performance as shown in the typical application
schematic. Please refer to “External Schottky Diode”
subsection in “Application Information” for more details.
During discontinuous mode, the zero crossing
comparator and the current limit comparator are turned
off. The bias current of most circuits are reduced. As a
result, the total power supply current during
discontinuous mode is only about 450μA, allowing the
MIC2165 to achieve high efficiency in light load
applications.
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 MIC2165 implements an internal digital soft-start by
making the 0.8V reference voltage V_REF ramp from 0 to
100% in about 5ms. Therefore, the output voltage is
controlled to increase slowly by a stair-case V_REF ramp.
Once the soft-start cycle ends, the related circuitry is
disabled to reduce current consumption. During soft-start,
the discontinuous mode is disabled in MIC2165.
Current Limit
The MIC2165 uses the R_DS(ON) of the low-side power
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
Figure 5. MIC2165 Current Limiting Circuit
Using the typical V_CL value of 133mV, the current limit value
is roughly estimated as:
I_CL
≈
133mV
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 current and that there is a
blanking delay of approximately 150ns.
September 2010
13
M9999-092410-E