MIC2130 Datasheet, PDF (11/20 Pages) Micrel Semiconductor – High Voltage Synchronous Buck Control IC with Low EMI Option

Micrel, Inc.
be disabled during an over current condition.
Under Voltage
A ±6% comparator monitors the output voltage and will
initiate the fast hysteretic control loop (FHyCL) to
regulate the output. A comparator monitors the output
voltage and sets PGOOD true when the output reaches
90% of the regulated output.
Over Voltage
If the voltage at the FB pin is detected to be 15% higher
than nominal for >2µs, then the controller is stopped
from switching immediately and latched off. Switching
can be re-started by taking EN below the channel’s
enable threshold and re-enabling or re-cycling power to
the IC.
Figure 5.
During the normal operation of a synchronous Buck
regulator, as the lower MOSFET is switched on, its drain
voltage will become negative with respect to ground as
the inductor current continues to flow from Source-to-
Drain. This negative voltage is proportional to output
load current, inductor ripple current and MOSFET
RDS_ON.
MIC2130/1
The larger the inductor current, the more negative VDS
becomes. This is utilized for the detection of over current
by passing a known fixed current source (200µA)
through a resistor R_CS which sets up an offset voltage
such that when 200µA x R_CS = I_DRAIN x RDS_ON the
MIC2130/31’s over current trigger is set. This disables
the next high side gate drive pulse. After missing the
high side pulse, the over current (OC) trigger is reset. If
on the next low side drive cycle, the current is still too
high i.e., V_CS is ≤ 0V, another high side pulse is missed
and so on. Thus reducing the overall energy transferred
to the output and V_OUT starts to fall. As this successive
missing of pulses results in an effectively lower switching
frequency, power inductor ripple currents can get very
high if left unlimited. The MIC2130/31 therefore limits
Duty Cycle during current limit to prevent currents
building up in the power inductor and output capacitors.
Current Limit Setting
The Simple Method
R_CS = I_OUT x RDS_ON(max)/200µA.
Accurate Method
For designs where ripple current is significant when
compared to I_OUT, or for low duty cycle operation,
calculating the current setting resistor R_CS should take
into account that one is sensing the peak inductor
current and that there is a blanking delay of
approximately 100ns.
April 2008
Figure 6.
Figure 7.
I _PK
= I_OUT
+ ^IRIPPLE
2
I _RIPPLE
= ^{VOUT ⋅ (1 − D)}
F_SWITCH ⋅ L
I _SET
= I_PK
− ^{VOUT ⋅ TDLY}
L
R_CS
=
I _SET
⋅ RDS_ON(max)
I_CS(min)
D = Duty Cycle
F_SWITCH = Switching Frequency
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M9999-042108-C