MIC2166 Datasheet, PDF (10/26 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller

Micrel, Inc.
MIC2166
Functional Description
The MIC2166 is an adaptive on-time buck controller built
for low cost and high performance. Featuring an internal
5V linear regulator and PGOOD output, it is designed for
a wide input voltage range from 4.5V to 28V, high output
power buck converters. An estimated ON-time method is
used in the MIC2166 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. 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 MIC2166 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
MIC2166 feedback pin FB via the voltage divider. The
FB voltage V_FB is compared to a 0.8V reference voltage
V_REF at the error comparator through a low gain
transconductance (g_m) amplifier at switching frequency.
This g_m amplifier improves the MIC2166 converter output
voltage regulation. If the FB voltage V_FB 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 T_ON Estimator” circuitry:
T_{ON(estimated)}
=
V_OUT
V_IN × f_sw
(1)
where V_OUT is the output voltage, V_IN is the power stage
input voltage, and f_SW is the switching frequency
(600kHz for MIC2166).
After an ON-time period, the MIC2166 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 on
V_FB in most cases. When V_FB 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 V_FB is less than the minimum OFF
time T_OFF(min), which is about 300ns typical, then the
MIC2166 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 300ns
T_OFF(min):
D_MAX
=
T_S − T_OFF(min)
T_S
= 1− ^300ns
T_S
where T_S = 1/f_SW. It is not recommended to use MIC2166
with a OFF-time close to T_OFF(min) during steady state
operation.
June 2010
The estimated ON-time method results in a constant
switching frequency in the MIC2166. The actual ON-time
varies slightly with the different rising and falling times of
the external MOSFETs. Therefore, the type of the
external MOSFETs and the output load current will
modify the actual ON-time and the switching frequency.
Also, the minimum T_ON results in a lower switching
frequency in high V_IN and low V_OUT applications, such as
24V to 1.0V. The minimum T_ON measured on the
MIC2166 evaluation board is about 100ns. 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 V_FB. Figure 2 shows the
MIC2166 control loop timing during steady-state
operation. During steady-state, the gm amplifier senses
V_FB ripple, which is proportional to the output voltage
(V_OUT) 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 V_FB.
At the valley of V_FB 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. MIC2166 Control Loop Timing
Figure 3 shows the load transient operation of the
MIC2166 converter. The output voltage drops due to the
sudden load increase, which causes V_FB to be less than
V_REF. This will cause the error comparator to trigger an
ON-time period. At the end of the ON-time period, a
minimum OFF-time T_OFF(min) is generated to charge C_BST
since V_FB is still below V_REF. Then, the next ON-time
period is triggered due to the low V_FB. Therefore, the
switching frequency changes during the load transient.
With the varying duty-cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small in MIC2166 converter.
10
M9999-060810-B