MIC26903 MICREL [Micrel Semiconductor], MIC26903 Datasheet - Page 15

• Manufacturer Part Number: MIC26903
• Description: 28V, 9A Hyper Light Load Synchronous DC/DC Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Functional Description
The MIC26903 is an adaptive ON-time synchronous
step-down DC/DC regulator with an internal 5V linear
regulator and a Power Good (PG) output. It is designed
to operate over a wide input voltage range from 4.5V to
28V and provides a regulated output voltage at up to 7A
of output current. An adaptive ON-time control scheme is
employed in 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
The MIC26903 is able to operate in either continuous
mode or discontinuous mode. The operating mode is
determined by the output of the Zero Cross comparator
(ZC) as shown in Figure 1.
Continuous Mode
In continuous mode, the output voltage is sensed by the
MIC26903 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
300ns, the MIC26903 control logic will apply the t
instead. t
the boost capacitor (C
MOSFET.
July 2011
the
t
OUT
OFF(min)
ON(estimat
error
is the output voltage and V
is required to maintain enough energy in
ed)
comparator
m
=
amplifier is below 0.8V, the ON-time
m
) amplifier. If the feedback voltage
V
IN
×
BST
V
600kHz
OUT
) to drive the high-side
through
OFF(min)
m
amplifier is below
, which is about
IN
a
is the power
low
OFF(min)
Eq. 1
gain
REF
ON
15
The maximum duty cycle is obtained from the 300ns
t
where t
It is not recommended to use MIC26903 with a OFF-time
close to t
V
and reduce the line regulation performance. Therefore,
the maximum output voltage of the MIC26903 should be
limited to 5.5V and the maximum external ripple injection
should be limited to 200mV. 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 V
results in a lower switching frequency in high V
applications, such as 24V to 1.0V. The minimum t
measured on the MIC26903 evaluation board is about
100ns. During load transients, the switching frequency is
changed due to the varying OFF-time.
To illustrate the control loop operation, we will analyze
both the steady-state and load transient scenarios.
Figure 2 shows the MIC26903 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.
OFF(min)
OUT
increases, the internal ripple injection will increase
:
S
D
= 1/600kHz = 1.66 μ s.
OFF(min)
max
=
t
S
during steady-state operation. Also, as
-
t
OFF(min)
t
S
FB
DD
falls below V
voltage. Also, the minimum t
=
1
-
300ns
t
S
ON
REF
, the OFF period
M9999-071311-A
estimator. The
MIC26903
IN
to V
Eq. 2
OUT
ON
ON
m