MIC2176-1 MICREL [Micrel Semiconductor], MIC2176-1 Datasheet - Page 14

• Manufacturer Part Number: MIC2176-1
• Description: Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Functional Description
The MIC2176 is an adaptive on-time synchronous buck
controller family built for high-input voltage and low
output voltage applications. It is designed to operate
over a wide input voltage range from, 4.5V to 75V and
the output is adjustable with an external resistive divider.
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
MOSFET’s R
start, enable, UVLO, and thermal shutdown.
Theory of Operation
Figure 1 illustrates the block diagram of the MIC2176.
The output voltage is sensed by the MIC2176 feedback
pin FB via the voltage divider R1 and R2, and compared
to a 0.8V reference voltage V
through a low-gain transconductance (g
the feedback voltage decreases and the amplifier output
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
Estimator” circuitry:
where V
input voltage, and f
(100kHz for MIC2176-1, 200kHz for MIC2176-2, and
300kHz for MIC2176-3).
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
360ns, the MIC2176 control logic will apply the t
instead. t
the boost capacitor (C
MOSFET.
November 2010
t
OUT
ON(estimat
OFF(min)
is the output voltage, V
is
DS(ON)
is required to maintain enough energy in
implemented
ed)
m
. The device features internal soft-
=
amplifier is below 0.8V, the ON-time
V
SW
IN
V
OUT
×
BST
is the switching frequency
f
SW
) to drive the high-side
REF
by
at the error comparator
OFF(min)
IN
sensing
is the power stage
, which is about
m
) amplifier. If
low-side
OFF(min)
(1)
ON
14
The maximum duty cycle is obtained from the 360ns
t
where t
with a OFF-time close to t
operation.
The adpative ON-time control scheme results in a
constant switching frequency in the MIC2176. The actual
ON-time and resulting switching frequency will vary with
the different rising and falling times of the external
MOSFETs. Also, the minimum t
switching frequency in high V
such as 48V to 1.0V. The minimum t
MIC2176 evaluation board is about 60ns. 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. For
easy analysis, the gain of the g
be 1. With this assumption, the inverting input of the
error comparator is the same as the feedback voltage.
Figure 2 shows the MIC2176 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 plus injected
voltage 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)
:
S
D
= 1/f
Figure 2. MIC2176 Control Loop Timing
max
SW
=
t
. It is not recommended to use MIC2176
S
−
t
OFF(min)
t
S
FB
falls below V
=
OFF(min)
1
−
m
IN
360ns
amplifier is assumed to
ON
t
to V
S
ON
during steady-state
ON
REF
results in a lower
measured on the
OUT
, the OFF period
M9999-111710-A
estimator. The
applications,
MIC2176
(2)
m