MIC2165_1011 Datasheet, PDF (11/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
Functional Description
The MIC2165 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 MIC2165 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 MIC2165 is an adaptive on-time buck controller.
Figure 1 illustrates the block diagram for the control loop.
MIC2165 is able to operate in two modes: continuous
mode and discontinuous mode. The operation mode of
MIC2165 is determined by the output of Zero Cross
Comparator (ZC), as shown in Figure 1.
Continuous Mode
In the continuous mode, the output voltage variation will
be sensed by the MIC2165 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
MIC2165 converter output voltage regulation. If the FB
voltage V_FB decreases and the output of the g_m 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 MIC2165).
After an ON-time period, the MIC2165 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
MIC2165 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 MIC2165
with an 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
MIC2165 should be limited to 5.5V. Please refer to “Setting
Output Voltage” subsection in “Application Information” for
more details.
The estimated ON-time method results in a constant
switching frequency in the MIC2165. 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 MIC2165 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 g_m 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 MIC2165 control loop
timing during steady-state operation in continuous mode.
During steady-state, the g_m 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.
September 2010
11
M9999-092410-E