MIC2174 Micrel Semiconductor, MIC2174 Datasheet - Page 10

• Manufacturer Part Number: MIC2174
• Description: Synchronous Buck Controller
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
Functional Description
The MIC2174 is an adaptive on-time synchronous buck
controller built for low cost and high performance. It is
designed for wide input voltage range from 3V to 40V
and for high output power buck converters. An
estimated-ON-time method is applied in MIC2174 to
obtain a constant switching frequency and to simplify the
control compensation. The over-current protection is
implemented without the use of an external sense
resistor. It 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 MIC2174 is an adaptive on-time buck controller.
Figure 1 illustrates the block diagram for the control loop.
The output voltage variation will be sensed by the
MIC2174 feedback pin FB via the voltage divider R1 and
R2, and compared to a 0.8V reference voltage V
the
transconductance (gm) amplifier, which improves the
MIC2174 converter output voltage regulation. If the FB
voltage decreases and the output of the gm amplifier is
below 0.8V, the error comparator will trigger the control
logic and generate an ON-time period, in which DH pin is
logic high and DL pin is logic low. The ON-time period
length
ESTIMATION” circuitry:
where V
stage input voltage.
After ON-time period, the MIC2174 goes into the OFF-
time period. In which DH pin is logic low and DL pin is
logic high. The OFF-time period length depends upon
the FB voltage in most cases. When the FB voltage
decreases and the output of the gm amplifier is below
0.8V, the ON-time period is trigger and the OFF-time
period ends. If the OFF-time period decided by the FB
voltage is less than the minimum OFF time T
which is about 363ns typical, the MIC2174 control logic
will apply the T
BST charging. The maximum duty cycle is obtained from
the 363ns T
where Ts = 1/300kHz = 3.33 μ s. It is not recommended to
use MIC2174 with a OFF time close to T
steady state.
The power stage input voltage V
Ton Estimation block through a 6:1 divider and 5V
voltage clamper. Therefore, if the V
September 2009
error
Dmax
T
OUT
is
ON(estimat
OFF(min)
is the output voltage, V
predetermined
comparator
=
OFF(min)
T
ed)
:
S
−
=
instead. T
T
T
V
OFF(min)
S
HSD
V
×
OUT
through
300kHz
by
=
OFF(min)
HSD
1
−
the
is fed into the Fixed
363ns
HSD
T
HSD
is required by the
S
a
is higher than
“FIXED
is the power
OFF(min)
low
OFF(min)
at the
REF
gain
T
(1)
ON
at
,
10
30V, the Fixed T
estimate Ton instead of the real V
switching frequency will be less than 300kHz:
The estimated-ON-time method results in a constant
300kHz switching frequency up to 30V V
ON time is varied with the different rising and falling time
of the external MOSFETs. Therefore, the type of the
external MOSFETs, the output load current, and the
control circuitry power supply V
ON time and the switching frequency. Also, the minimum
Ton results in a lower switching frequency in the high
V
application. The minimum Ton measured on the
MIC2174 evaluation board with Si7148DP MOSFETs is
about 184ns. During the load transient, the switching
frequency is changed due to the varying OFF time.
To illustrate the control loop, the steady-state scenario
and the load transient scenario are analyzed. For easy
analysis, the gain of the gm amplifier is assumed to be 1.
With this assumption, the inverting input of the error
comparator is the same as the FB voltage. Figure 2
shows the MIC2174 control loop timing during the
steady-state. During the steady-state, the gm amplifier
senses the FB voltage ripple, which is proportional to the
output voltage ripple and the inductor current ripple, to
trigger
predetermined by the estimation. The ending of OFF
time is controlled by the FB voltage. At the valley of the
FB voltage ripple, which is below than V
ends and the next ON-time period is triggered through
the control logic circuitry.
Figure 3 shows the load transient scenario of the
MIC2174 converter. The output voltage drops due to the
sudden load increasing, which would cause the FB
voltage to be less than V
HSD
and low V
f
the
SW(VHDS
Figure 2. MIC2174 Control Loop Timing
ON-time
>
30V)
OUT
ON
=
applications, such as 36V to 1.0V
V
30V
Estimation block uses 30V to
HSD
period.
REF
×
. This will cause the error
300kHz
www.DataSheet4U.com
IN
will modify the actual
The
HSD
. As a result, the
M9999-090409-B
REF
HSD
ON
, OFF period
. The actual
MIC2174
time
(2)
is