MIC26950 Datasheet, PDF (13/27 Pages) Micrel Semiconductor – 12A Hyper Speed Control Synchronous DC-DC Buck Regulator

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 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 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_REF
at the error comparator through a low gain
transconductance (g_m) amplifier. If the feedback voltage
decreases and the output of the g_m amplifier is below
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_ON
ESTIMATION” circuitry:
t _{ON(estimated)}
=
V_IN
V_OUT
× 300kHz
(1)
where V_OUT is the output voltage and V_IN is the power
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_m amplifier is below 0.8V, the ON-time
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_OFF(min), which is about
360ns, the MIC26950 control logic will apply the t_OFF(min)
instead. t_OFF(min) is required to maintain enough energy in
the boost capacitor (C_BST) to drive the high-side
MOSFET. The maximum duty cycle is obtained from the
360ns t_OFF(min):
D_max
=
t_S
− t _OFF(min)
t_S
= 1− ^360ns
t_S
(2)
MIC26950
where t_S = 1/300kHz = 3.33μs. It is not recommended to
use MIC26950 with a OFF-time close to t_OFF(min) during
steady-state operation. Also, as V_OUT increases, the
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_ON
results in a lower switching frequency in high V_IN to V_OUT
applications, such as 26V to 1.0V. The minimum t_ON
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_m amplifier is assumed to
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_m
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_ON estimator. The
termination of the OFF-time is controlled by the feedback
voltage. At the valley of the feedback voltage ripple,
which occurs when V_FB falls below V_REF, the OFF period
ends and the next ON-time period is triggered through
the control logic circuitry.
Figure 2. MIC26950 Control Loop Timing
September 2010
13
M9999-091710-C