MIC2174_10 Datasheet, PDF (11/27 Pages) Micrel Semiconductor – Synchronous Buck Controller Featuring Adaptive On-Time Control 40V Input, 300kHz

Micrel, Inc.
MIC2174/MIC2174C
Functional Description
The MIC2174/MIC2174C is an adaptive on-time
synchronous buck controller built for low cost and high
performance. It is designed for a wide input voltage
range from 3V to 40V and for high output power buck
converters. An estimated-ON-time method is applied in
MIC2174/MIC2174C 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/MIC2174C is an adaptive on-time
synchronous buck controller. Further, Figure 1 illustrates
the block diagram for the control loop. The output
voltage variation will be sensed by the
MIC2174/MIC2174C 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 (gm) amplifier, which improves the
MIC2174/MIC2174C converter output voltage regulation.
If the FB voltage decreases and the output of the gm
amplifier is below 0.8V, then the error comparator will
trigger the control logic and generate an ON-time period,
where in DH pin is logic high and DL pin is logic low. The
ON-time period length is predetermined by the “FIXED
T_ON ESTIMATION” circuitry:
T_{ON(estimated)}
=
V_OUT
V_HSD × 300kHz
(1)
where V_OUT is the output voltage, V_HSD is the power
stage input voltage.
After an ON-time period, the MIC2174/MIC2174C goes
into the OFF-time period. This is when the 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, then the ON-time period is
triggered and the OFF-time period ends. If the OFF-time
period determined by the FB voltage is less than the
minimum OFF time T_OFF(min), which is about 363ns
typical, then the MIC2174/MIC2174C 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.
The maximum duty cycle is obtained from the 363ns
T_OFF(min):
Dmax = ^{TS − TOFF(min)} = 1 − ^363ns
T_S
T_S
where Ts = 1/300kHz = 3.33μs. It is not recommended to
use MIC2174/MIC2174C with a OFF-time close to
T_OFF(min) during steady-state operation. Also, as V_OUT
increases, the internal ripple injection will increase and
reduce the line regulation performance. Therefore, the
maximum output voltage of the MIC2174 should be
limited to 5.5V for up to 28V V_HSD and 3.6V for V_HSD
higher than 28V. If a higher output voltage is required,
use the MIC2176 instead. Please refer to “Setting Output
Voltage” subsection in “Application Information” for more
details.
The power stage input voltage V_HSD is fed into the Fixed
T_ON Estimation block through a 6:1 divider and 5V
voltage clamper. Therefore, if the V_HSD is higher than
30V, then the Fixed T_ON Estimation block uses 30V to
estimate T_ON instead of the real V_HSD. As a result, the
switching frequency will be less than 300kHz:
f_{SW(VHDS >30V)}
=
30V
V_HSD
× 300kHz
(2)
The estimated ON-time method results in a constant
300kHz switching frequency up to 30V V_HSD. The actual
ON-time varies with the different rising and falling times
of the external MOSFETs. Therefore, the type of the
external MOSFETs, the output load current, and the
control circuitry power supply V_IN will modify the actual
ON-time and the switching frequency. Also, the minimum
T_ON results in a lower switching frequency in high V_HSD
and low V_OUT applications, such as 36V to 1.0V. The
minimum T_ON measured on the MIC2174/MIC2174C
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/MIC2174C control loop timing
during steady-state operation. During 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 the ON-time period. The ON-
time is predetermined by the estimation. The ending of
OFF-time is controlled by the FB voltage. At the valley of
September 2010
11
M9999-091310-C