MIC261203 Datasheet, PDF (15/31 Pages) Micrel Semiconductor – 28V, 12A Hyper Light Load™ Synchronous DC/DC Buck Regulator

Micrel, Inc.
Functional Description
The MIC261203 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 MIC261203 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
MIC261203 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
× 600kHz
Eq. 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
300ns, then the MIC261203 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.
MIC261203
The maximum duty cycle is obtained from the 300ns
t_OFF(min):
D_max
=
t_S
- t_OFF(min)
t_S
= 1-
300ns
t_S
Eq. 2
where t_S = 1/600kHz = 1.66μs.
It is not recommended to use MIC261203 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
MIC261203 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_DD voltage. Also, the minimum t_ON
results in a lower switching frequency in high V_IN to V_OUT
applications, such as 24V to 1.0V. The minimum t_ON
measured on the MIC261203 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, both the steady-
state and load transient scenarios will be analyzed.
Figure 2 shows the MIC261203 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.
July 2011
15
M9999-071311-A