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MIC28510_12 Datasheet, PDF (15/30 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control Synchronous DC/DC Buck Regulator | |||
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Micrel, Inc.
Functional Description
The MIC28510 is an adaptive ON-time synchronous
step-down DC/DC regulator. It is designed to operate
over a wide input voltage range from, 4.5V to 75V, and
provides a regulated output voltage at up to 4A of output
current. A digitally-modified adaptive ON-time control
scheme is employed in order to obtain a constant-
switching frequency and to simplify the control
compensation. Over current protection is implemented
without the use of an external sense resistor. The device
includes an internal soft-start function which reduces the
power supply input surge current at start-up by
controlling the output voltage rise time.
Theory of Operation
Figure 1 illustrates the block diagram for the control loop
of the MIC28510. The output voltage is sensed by the
MIC28510 feedback pin FB via the voltage divider R1
and R2, and compared to a 0.8V reference voltage VREF
at the error comparator through a lowâgain
transconductance (gm) amplifier. If the feedback voltage
decreases and the output of the gm amplifier is below
0.8V, then the error comparator will trigger the control
logic and generate an ONâtime period. The ONâtime
period length is predetermined by the âFIXED tON
ESTIMATIONâ circuitry:
tON(estimated)
ï½
VOUT
VIN ï´ fSW
Eq. 1
where VOUT is the output voltage and VIN is the power
stage input voltage and fSW is the switching frequency.
At the end of the ONâtime period, the internal highâside
driver turns off the highâside MOSFET and the lowâside
driver turns on the lowâside MOSFET. The OFFâtime
period length depends upon the feedback voltage in
most cases. When the feedback voltage decreases and
the output of the gm amplifier is below 0.8V, the ON-time
period is triggered and the OFFâtime period ends. If the
OFFâtime period determined by the feedback voltage is
less than the minimum OFFâtime tOFF(MIN), which is about
360ns, then the MIC28510 control logic will apply the
tOFF(MIN) instead. The minimum tOFF(MIN) period is required
to maintain enough energy in the boost capacitor (CBST)
to drive the highâside MOSFET.
MIC28510
The maximum duty cycle is obtained from the 360ns
tOFF(MIN):
DMAX
ï½
tS
ï t OFF(MIN)
tS
ï½ 1 ï 360ns
tS
Eq. 2
where tS = 1/fSW. It is not recommended to use
MIC28510 with a OFF-time close to tOFF(MIN) during
steady-state operation.
The actual ONâtime and resulting switching frequency
will vary with the partâtoâpart variation in the rise and fall
times of the internal MOSFETs, the output load current,
and variations in the VDD voltage. Also, the minimum tON
results in a lower switching frequency in high VIN to VOUT
applications, such as 75V to 1.0V.
Figure 2 shows the MIC28510 control loop timing during
steady-state operation. During steady-state, the gm
amplifier senses the feedback voltage ripple, which is
proportional to the output voltage ripple and the inductor
current ripple, to trigger the ONâtime period. The ONâ
time is predetermined by the tON estimator. The
termination of the OFFâtime is controlled by the
feedback voltage. At the valley of the feedback voltage
ripple, which occurs when VFB falls below VREF, the OFF
period ends and the next ONâtime period is triggered
through the control logic circuitry.
Figure 2. MIC28510 Control Loop Timing
March 2012
15
M9999-030912-A
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