MIC2164 MICREL [Micrel Semiconductor], MIC2164 Datasheet - Page 12

• Manufacturer Part Number: MIC2164
• Description: Constant Frequency, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Functional Description
The MIC2164/-2/-3 is a adaptive on-time synchronous
buck controller family built for low cost and high
performance. They are designed for wide input voltage
range from 3V to 28V and for high output power buck
converters. An estimated-ON-time method is applied in
MIC2164/-2/-3 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 MIC2164/-2/-3 is a adaptive on-time buck controller
family. Figure 1 illustrates the block diagram for the
control loop. The output voltage variation will be sensed
by the MIC2164/-2/-3 feedback pin FB via the voltage
divider R1 and R2, and compared to a 0.8V reference
voltage V
transconductance (gm) amplifier, which improves the
MIC2164/-2/-3 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 is predetermined by the “FIXED TON
ESTIMATION” circuitry:
where V
stage input voltage, and f
(300kHz for MIC2164, 600kHz for MIC2164-2, and
1MHz for MIC2164-3).
After ON-time period, the MIC2164/-2/-3 goes into the
OFF-time period, in which DH pin is logic low and DL pin
is logic high. The OFF-time period length is depending
on 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 MIC2164/-2/-3 control
logic will apply the T
by the BST charging. The maximum duty cycle is
obtained from the 363ns T
where Ts = 1/f
MIC2164/-2/-3 with a OFF time close to T
steady state.
September 2009
Dmax
T
OUT
REF
ON(estimat
is the output voltage, V
at the error comparator through a low gain
=
T
SW
S
ed)
. It is not recommended to use
−
OFF(min)
=
T
T
OFF(min)
S
V
HSD
OFF(min)
V
SW
OUT
instead. T
is the switching frequency
⋅
f
=
sw
:
1
−
363ns
T
OFF(min)
HSD
S
is the power
OFF(min)
is required
OFF(min)
at the
(1)
,
12
The estimated-ON-time method results in a constant
switching frequency in MIC2164/-2/-3. The actual 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
MIC2164-3 application. The minimum Ton measured on
the MIC2164 evaluation board is about 138ns. 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 MIC2164/-2/-3 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
MIC2164/-2/-3 converter. The output voltage drops due
to the sudden load increasing, which would cause the
FB voltage to be less than V
comparator to trigger ON-time period. At the end of the
ON-time period, a minimum OFF time T
generated to charge BST since the FB voltage is still
below the V
triggered due to the low FB voltage. Therefore, the
switching frequency changes during the load transient.
HSD
and low V
Figure 2. MIC2164/-2/-3 Control Loop Timing
the
REF
ON-time
OUT
. Then, the next ON-time period is
applications, such as 24V to 1.0V
period.
REF
. This will cause the error
IN
will modify the actual
The
M9999-090409-B
REF
MIC2164/-2/-3
ON
, OFF period
OFF(min)
time
is
is