MIC26903ZA_14 Datasheet, PDF (15/29 Pages) Micrel Semiconductor – 28V, 9A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Functional Description
The MIC26903-ZA is an adaptive ON-time synchronous
step-down DC-to-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
used to get a constant switching frequency and to
simplify the control compensation. Overcurrent protection
is implemented without using an external sense resistor.
The device includes an internal soft-start function that
reduces the power supply input surge current at start-up
by controlling the output voltage rise time.
Theory of Operation
The MIC26903-ZA operates in a continuous mode as
shown in the Functional Diagram.
Continuous Mode
In continuous mode, the output voltage is sensed by the
MIC26903-ZA feedback pin FB through the voltage
divider R1 and R2. It is then compared to a 0.6V
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 0.6V, 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_OUT
V_IN × 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 0.6V, 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, the MIC26903-ZA 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.
MIC26903-ZA
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.
Using the MIC26903-ZA with an OFF-time close to
t_OFF(min) during steady-state operation is not
recommended. Also, as V_OUT increases, the internal
ripple injection increases and reduces the line regulation
performance. Therefore, the maximum output voltage of
the MIC26903-ZA should be limited to 5.5V and the
maximum external ripple injection should be limited to
200mV. Please refer to the “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 MIC26903-ZA evaluation board is about
100ns. During load transients, the switching frequency is
changed because of the varying OFF-time.
To illustrate the control loop operation, the datasheet will
discuss both the steady-state and load transient
scenarios. Figure 1 shows the MIC26903-ZA control loop
timing during steady-state operation. During steady-state
operation, 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-time period ends and the next ON-time period is
triggered through the control logic circuitry.
July 22, 2014
15
Revision 1.1