MIC28512 Datasheet, PDF (20/34 Pages) Microchip Technology – 70V, 2A Synchronous Buck Regulator

MIC28512
I_L CROSSES 0 AND V_FB > 0.8.
DISCONTINUOUS MODE STARTS
I_L
V_FB < 0.8. WAKEUP FROM
DISCONTINUOUS MODE.
0
V_FB
V_REF
ZC
V_HSD
V_LSD
ESTIMATED ON-TIME
FIGURE 4-4:
MIC28512-1 Control Loop
Timing (Discontinuous Mode).
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
450 μA, allowing the MIC28512-1 to achieve high
efficiency in light load applications.
4.3 V_DD Regulator
The MIC28511 provides a 5V regulated V_DD to bias
internal circuitry for V_IN ranging from 5.5V to 70V.
When V_IN is less than 5.5V, V_DD should be tied to V_IN
pins to bypass the internal linear regulator.
4.4 Soft-Start
Soft-start reduces the power supply inrush current at
startup by controlling the output voltage rise time while
the output capacitor charges.
The MIC28511 implements an internal digital soft-start
by ramping up the 0.8V reference voltage (V_REF) from
0 to 100% in about 5 ms with 9.7 mV steps. This
controls the output voltage rate of rise at turn on,
minimizing inrush current and eliminating output
voltage overshoot. Once the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption.
DS20005524A-page 20
4.5 Current Limit
The MIC28512 uses the R_DS(ON) of the internal
low-side power MOSFET to sense overcurrent
conditions. In each switching cycle, the inductor current
is sensed by monitoring the low-side MOSFET during
its ON period. The sensed voltage, V_(ILIM), is compared
with the power ground (PGND) after a blanking time of
150 ns. The voltage drop of the resistor R_ILIM is
compared with the low-side MOSFET voltage drop to
set the overcurrent trip level. The small capacitor
connected from the ILIM pin to PGND can be added to
filter the switching node ringing, allowing a better short
limit measurement. The time constant created by R_ILIM
and the filter capacitor should be much less than the
minimum off time.
The overcurrent limit can be programmed by using
Equation 4-3:
EQUATION 4-3:
R_ILIM
=
_---I_{--C----L---I-M------–----0---.--5----}_-----_----I_{--L-----P---P-----}_----_-----R_{----D---S-----O----N------+-----}V_----C---L-
I_CL
Where:
I_CLIM
R_DS(ON)
V_CL
I_CL
∆I_L(PP)
Desired Current Limit
On-Resistance of Low-Side Power
MOSFET (40 mΩ typical)
Current-limit threshold.
14 mV (typical absolute value).
See the Electrical Characteristics table.
Current-limit source current.
70µA (typical). See the Electrical Char-
acteristics table.
Inductor Current Peak-to-Peak
Use Equation 4-4 to calculate the
inductor ripple current
The peak-to-peak inductor current ripple is calculated
with Equation 4-4.
EQUATION 4-4:
I _{L  P P }
=
V_{----O----U----T----}_-----_--V_{----I--N-----M-----A---X------–-----}V_{---O----U----T---}
V_{INMAX}  f_SW  L
The MOSFET R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to use the
R_DS(ON) at maximum junction temperature with a 20%
margin to calculate R_ILIM in Equation 4-3.
In case of hard short, the current-limit threshold 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 capacitor during soft-start is under the folded
short limit; otherwise the supply will go into hiccup
mode and may not be finishing the soft-start
successfully.
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