MIC28500 Datasheet, PDF (14/29 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control™ Synchronous DC-DC Buck Regulator

Micrel, Inc.
Functional Description
The MIC28500 is an adaptive ON-time synchronous
step-down DC-DC regulator. It is designed to operate
over a wide input voltage range from, 30V to 75V, and
provides a regulated output voltage at up to 4A 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 MIC28500. The output voltage is sensed by the
MIC28500 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_OUT
V_IN × f_SW
Eq. 1
where V_OUT is the output voltage and V_IN is the power
stage input voltage and f_SW is the switching frequency.
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, then the MIC28500 control logic will apply the
t_OFF(min) instead. The minimum t_OFF(min) period 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
Eq. 2
MIC28500
where t_S = 1/f_SW. It is not recommended to use
MIC28500 with a OFF-time close to t_OFF(min) during
steady-state operation..
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 75V to 1.0V. The minimum t_ON
measured on the MIC28500 evaluation board is about
184ns. During load transients, the switching frequency is
changed due to the varying OFF-time.
Figure 2 shows the MIC28500 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. MIC28500 Control Loop Timing
June 2011
14
M9999-060311-B