MIC2101 Datasheet, PDF (22/37 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
During discontinuous mode, the bias current of most
circuits are reduced. As a result, the total power supply
current during discontinuous mode is only about 400μA,
allowing the MIC2101 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 MIC2101/02 implements an internal digital soft-start
by making the 0.8V reference voltage V_REF ramp from 0
to 100% in about 6ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V_FB ramp. Once the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption. V_DD must be powered up at the same time
or after V_IN to make the soft-start function correctly.
Current Limit
The MIC2101/02 uses the R_DS(ON) and external resistor
connected from ILIM pin to SW node to decides the
current limit.
Figure 4. MIC2101/02 Current Limiting Circuit
In each switching cycle of the MIC2101/02 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. The sensed voltage V_(ILIM) is
compared with the power ground (PGND) after a
blanking time of 150nS. In this way the drop voltage over
the resistor R_CL (V_CL) is compared with the drop over the
bottom FET generating the short current limit.
MIC2101/02
The small capacitor (C_CL) connected from ILIM pin to
PGND filters the switching node ringing during the off
time allowing a better short limit measurement. The time
constant created by R_CL and C_CL should be much less
than the minimum off time.
The V_CL drop allows programming of short limit through
the value of the resistor (R_CL), If the absolute value of the
voltage drop on the bottom FET is greater than V_CL’ in
that case the V_(ILIM) is lower than PGND and a short
circuit event is triggered. A hiccup cycle to treat the short
event is generated. The hiccup sequence including the
soft start reduces the stress on the switching FETs and
protects the load and supply for severe short conditions.
The short circuit current limit can be programmed by
using the formula illustrated in Equation 3:
R _CL

(I_CLIM
 ∆_PP
 0.5) R_DS(ON)
I_CL
 V_CL
Eq. 3
Where I_SH = Desired current limit
∆_PP = Inductor current peak-to-peak
R_{DS (ON)} = On-resistance of low-side power MOSFET
V_CL = Current-limit threshold, the typical value is 14mV in
EC table
I_CL = Current-limit source current, the typical value is
80µA in EC table.
In case of hard short, the short limit is folded down to
allow an indefinite hard short on the output without any
destructive effect. It is mandatory to make sure that the
inductor current used to charge the output capacitance
during soft start is under the folded short limit, otherwise
the supply will go in hiccup mode and may not be
finishing the soft start successfully.
The MOSFET R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a 50%
margin to I_CL in the above equation to avoid false current
limiting due to increased MOSFET junction temperature
rise. It is also recommended to connect SW pin directly
to the drain of the low-side MOSFET to accurately sense
the MOSFETs R_DS(ON).
August 2012
22
M9999-080712-A