MIC2124 Datasheet, PDF (10/24 Pages) Micrel Semiconductor – Constant Frequency, Synchronous Current Mode Buck Controller

Micrel, Inc.
MIC2124
until V_LX > -127mV, and then goes into the ON status
with minimum ON-time. The current limit threshold V_CL
has a fold back characteristic 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 MIC2124 current limiting circuit.
Figure 3. MIC2124 Load-Transient Response
Unlike in current-mode control, the MIC2124 uses
adaptive ON-time current mode control. The MIC2124
predetermined ON-time control loop has the advantage
of constant ON-time mode control and eliminates the
need for the slope compensation.
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 MIC2124 implements an internal digital soft-start by
making the 0.8V reference voltage V_REF ramp from 0 to
100% in about 4ms. 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. V_IN must be
powered up no earlier than V_HSD to make the soft-start
function behavior correctly.
Current Limit
The MIC2174/MIC2174C uses the RSD(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 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 MIC2124 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. The sensed voltage V_LX is
compared with a current-limit threshold voltage V_CL after
a blanking time of 150ns. If the sensed voltage V_LX is
under V_CL, which is -127mV typical at 0.8V feedback
voltage, the MIC2124 keeps the low-side MOSFET on
Figure 4. MIC2124 Current Limiting Circuit
Using the typical V_CL value of -127mV, the current limit
value in the inductor is roughly estimated as:
I_CL
≈
127mV
R_DS(ON)
For designs where the inductor current ripple is
significant compared to the load current I_OUT, or for low
duty cycle operation, calculating the load current limit
I_CL(LOAD) should take into account that one is sensing the
peak inductor current.
I_CL(LOAD)
=
127mV
R_DS(ON)
−
ΔI_L(pp)
2
(2)
ΔI_L(pp)
=
V_OUT × (1− D)
f _SW ×L
(3)
where:
V_OUT = The output voltage
ΔI_L(pp) = Inductor current ripple peak-to-peak value
D = Duty Cycle
f_SW = Switching frequency
The MOSFET R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add 50%
margin to I_CL(LOAD) in the above equation to avoid false
current limiting due to increased MOSFET junction
temperature rise. It is also recommended to connect LX
pin directly to the drain of the low-side MOSFET to
accurately sense the MOSFETs R_DS(ON)
June 2010
10
M9999-060810-D