MIC26950 MICREL, MIC26950 Datasheet - Page 13

• Manufacturer Part Number: MIC26950
• Description: 12A Hyper Speed Control Synchronous DC-DC Buck Regulator
• Manufacturer: MICREL

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Micrel, Inc.
Functional Description
The MIC26950 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 26V and
provides a regulated output voltage at up to 12A of
output current. A digitally modified adaptive ON-time
control scheme is employed in to obtain a constant
switching
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 MIC26950. The output voltage is sensed by the
MIC26950 feedback pin FB via the voltage divider R1
and R2, and compared to a 0.8V reference voltage V
at
transconductance (g
decreases and the output of the g
0.8V, then the error comparator will trigger the control
logic and generate an ON-time period. The ON-time
period length is predetermined by the “FIXED t
ESTIMATION” circuitry:
where V
stage input voltage.
At the end of the ON-time period, the internal high-side
driver turns off the high-side MOSFET and the low-side
driver turns on the low-side MOSFET. The OFF-time
period length depends upon the feedback voltage in
most cases. When the feedback voltage decreases and
the output of the g
period is triggered and the OFF-time period ends. If the
OFF-time period determined by the feedback voltage is
less than the minimum OFF-time t
360ns, the MIC26950 control logic will apply the t
instead. t
the boost capacitor (C
MOSFET. The maximum duty cycle is obtained from the
360ns t
September 2010
the
OFF(min)
D
t
OUT
OFF(min)
ON(estimat
max
error
frequency
is the output voltage and V
:
=
t
is required to maintain enough energy in
S
ed)
comparator
−
m
t
amplifier is below 0.8V, the ON-time
m
=
OFF(min)
t
S
) amplifier. If the feedback voltage
V
and
IN
BST
×
V
300kHz
OUT
=
) to drive the high-side
1
to
−
through
360ns
simplify
OFF(min)
t
S
m
amplifier is below
, which is about
IN
a
is the power
the
low
control
OFF(min)
gain
REF
(1)
(2)
ON
13
where t
use MIC26950 with a OFF-time close to t
steady-state operation. Also, as V
internal ripple injection will increase and reduce the line
regulation performance. Therefore, the maximum output
voltage of the MIC26950 should be limited to 5.5V.
Please refer to “Setting Output Voltage” subsection in
“Application Information” for more details.
The actual ON-time and resulting switching frequency
will vary with the part-to-part variation in the rise and fall
times of the internal MOSFETs, the output load current,
and variations in the VDD voltage. Also, the minimum t
results in a lower switching frequency in high V
applications, such as 26V to 1.0V. The minimum t
measured on the MIC26950 evaluation board is about
184ns. During load transients, the switching frequency is
changed due to the varying OFF-time.
To illustrate the control loop operation, will be analyzed
both the steady-state and load transient scenarios. For
easy analysis, the gain of the g
be 1. With this assumption, the inverting input of the
error comparator is the same as the feedback voltage.
Figure 2 shows the MIC26950 control loop timing during
steady-state operation. During steady-state, the g
amplifier senses the feedback voltage ripple, which is
proportional to the output voltage ripple and the inductor
current ripple, to trigger the ON-time period. The ON-
time is predetermined by the t
termination of the OFF-time is controlled by the feedback
voltage. At the valley of the feedback voltage ripple,
which occurs when V
ends and the next ON-time period is triggered through
the control logic circuitry.
S
Figure 2. MIC26950 Control Loop Timing
= 1/300kHz = 3.33μs. It is not recommended to
FB
falls below V
m
amplifier is assumed to
ON
REF
OUT
M9999-091710-C
, the OFF period
estimator. The
increases, the
OFF(min)
MIC26950
IN
to V
during
OUT
ON
ON
m