MIC2125 Datasheet, PDF (19/34 Pages) Microchip Technology – 28V Synchronous Buck Controllers Featuring Adaptive ON-Time Control

4.4 Current Limit
The MIC2125/6 uses the low-side MOSFET R_DS(ON) to
sense the inductor current.
FIGURE 4-4:
Circuit
MIC2125/6 Current-Limiting
In each switching cycle of the MIC2125/6 converter, the
inductor current is sensed by monitoring the voltage
across the low-side MOSFET during the OFF period.
An internal current source of 36 µA generates a voltage
across the external resistor R_CL. The I_LIM pin voltage
V_(ILIM) is the sum of the voltage across the low side
MOSFET and the voltage across the resistor (V_CL).
The sensed voltage V_(ILIM) is compared with the power
ground (P_GND) after a blanking time of 150 ns.
If the absolute value of the voltage drop across the low
side MOSFET is greater than V_CL, the current limit
event is triggered. Eight consecutive current limit
events triggers hiccup mode. The hiccup sequence,
including the soft-start, reduces the stress on the
switching FETs and protects the load and supply from
severe short conditions.
The current limit can be programmed by using
Equation 4-3.
EQUATION 4-3:
R_CL
=
_---I_{--C----L---I-M------+-----}_{----P---P----}_{-----0---.--5---}_---_-----R_{----D----S-----O---N-------–----}V_{----O----F---F---S---E---T-}
I_CL
Where:
I_CLIM
∆_PP
R_DS(ON)
V_OFFSET
I_CL
Desired Current Limit
Inductor Current Peak-to-Peak
On-Resistance of Low-Side Power
MOSFET
Current-Limit Comparator Offset (Typical
Value is –4 mV per Table 1-1)
Current-Limit Source Current (Typical
Value is 36 µA, per Table 1-1)
MIC2125/6
Because MOSFET R_DS(ON) varies from 30% to 40%
with temperature, it is recommended to add a 50%
margin to I_CL in the previous equation to avoid false
current limiting due to increased MOSFET junction
temperature rise. It is also recommended to connect
the SW pin directly to the drain of the low-side
MOSFET to accurately sense the MOSFET’s R_DS(ON).
4.5 Negative Current Limit
(MIC2126 Only)
The MIC2126 implements negative current limit by
sensing the SW voltage when the low-side FET is off.
If the SW node voltage exceeds 12 mV typical, the
device turns off the low-side FET until the next ON-time
event is triggered. The negative current limit value is
given by Equation 4-4.
EQUATION 4-4:
Where:
I_NLIM
R_DS(ON)
I_NLIM
=
----1---2---^m-----^V-----
R_{DSON}
Negative Current Limit
On-Resistance of Low-Side Power
MOSFET
4.6 MOSFET Gate Drive
The MIC2125/6 high-side drive circuit is designed to
switch an N-Channel MOSFET. Figure 4-1 shows a
bootstrap circuit, consisting of a PMOS switch and
C_BST. This circuit supplies energy to the high-side drive
circuit. Capacitor C_BST is charged while the low-side
MOSFET is on and the voltage on the SW pin is
approximately 0V. When the high-side MOSFET driver
is turned on, energy from C_BST is used to turn the
MOSFET on. If the bias current of the high-side driver
is less than 10 mA, a 0.1 μF capacitor is sufficient to
hold the gate voltage within minimal droop, (i.e., ∆_BST
= 10 mA × 3.33 μs/0.1 μF = 333 mV). A small resistor,
RG in series with C_BST, can be used to slow down the
turn-on time of the high-side N-channel MOSFET.
4.7 Overvoltage Protection
The MIC2125/6 includes the OVP feature to protect the
load from overshoots due to input transients and output
short to a high voltage. When the overvoltage condition
is triggered, the converter turns off immediately to allow
the output voltage to discharge. The MIC2125/6 power
should be recycled to enable it again.
 2015 Microchip Technology Inc.
DS20005459B-page 19