MIC45208-1 Datasheet, PDF (15/35 Pages) Micrel Semiconductor – 26V 10A DC-to-DC Power Module

Micrel, Inc.
MIC45208
Figure 4. MIC45208-1 Control Loop Timing
(Discontinuous Mode)
During discontinuous mode, the bias current of most
circuits is substantially reduced. As a result, the total
power supply current during discontinuous mode is only
about 350µA, allowing the MIC45208-1 to achieve high
efficiency in light load applications.
Soft-Start
Soft-start reduces the input power supply surge current at
startup by controlling the output voltage rise time. The
input surge appears while the output capacitor is charged
up.
The MIC45208 implements an internal digital soft-start by
making the 0.8V reference voltage V_REF ramp from 0 to
100% in about 4ms 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.
PVDD must be powered up at the same time or after VIN
to make the soft-start function correctly.
Current Limit
The MIC45208 uses the R_DS(ON) of the low-side MOSFET
and external resistor connected from ILIM pin to SW
node to set the current limit.
Figure 5. MIC45208 Current-Limiting Circuit
In each switching cycle of the MIC45208, 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 R15
(V_CL) is compared with the drop over the bottom FET
generating the short current limit. The small capacitor
(C15) 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 R15 and C6 should be much less than the
minimum off time.
The V_CL drop allows programming of short limit through
the value of the resistor (R15). If the absolute value of the
voltage drop on the bottom FET becomes greater than
V_CL, and the V_ILIM falls below PGND, an over-current is
triggered causing the IC to enter hiccup mode. 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 Equation 3.
R15 = ^{(ICLIM − DIL(PP) × 0.5) × RDS(ON) + VCL}
I_CL
Eq. 3
Where:
I_CLIM = Desired current limit
R_DS(ON) = On-resistance of low-side power MOSFET, 6mΩ
typically.
V_CL = Current-limit threshold (typical absolute value is
14mV per the Electrical Characteristics table).
I_CL = Current-limit source current (typical value is 70µA,
per the Electrical Characteristics table).
February 6, 2015
15
Revision 2.0