MIC2124 Datasheet, PDF (9/24 Pages) Micrel Semiconductor – Constant Frequency, Synchronous Current Mode Buck Controller

Micrel, Inc.
Functional Description
The MIC2124 is an adaptive on-time current mode
synchronous buck controller built for low cost and high
performance. It is designed for wide input voltage range
from 3V to 18V and for high output power buck
converters. An estimated-ON-time method is applied in
MIC2124 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 MIC2124 is an adaptive on-time current mode buck
controller. Figure 1 illustrates the block diagram for the
control loop. The output voltage variation due to load or
line changes will be sensed by the inverting input of the
transconductance error amplifier via the feedback
resistors (R_FB1 and R_FB2 in “Typical Application”), and
compared to a reference voltage at the non-inverting
input. This will cause a small change in the DC voltage
level at the output of the error amplifier, or V_COMP.
Meanwhile, the inductor current is sensed through the
bottom MOSFET R_DS(ON) and “Bottom Current Sense
Circuit” as V_IL. If V_IL is lower than V_COMP, an ON-time
period is triggered, in which DH pin is logic high and DL
pin is logic low. The ON-time period length is
predetermined by the “Fixed Ton Estimator” 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 MIC2124 goes into the
OFF-time period, in which DH pin is logic low and DL pin
is logic high. The inductor current and V_IL decrease
during OFF time. If V_IL is above V_COMP, the OFF status is
maintained. When V_IL is below V_COMP, the ON-time
period is triggered and the OFF-time period ends. If the
OFF-time period determined by the inductor current and
V_COMP is less than the minimum OFF time T_OFF(min), which
is 350ns typical, the MIC2124 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 350ns T_OFF(min):
D_MAX
=
T_S − T_OFF(min)
T_S
= 1− ^350ns
T_S
where T_S = 1/300kHz = 3.33μs. It is not recommended to
use MIC2124 with a OFF-time close to T_OFF(min) during
steady state operation.
The estimated ON-time method results in a constant
June 2010
MIC2124
300kHz switching frequency. The actual ON-time varies
a little 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 slightly modify the actual
ON-time and the switching frequency. Also, the minimum
T_ON, which is 140ns typical, results in a lower switching
frequency in high V_HSD and low V_OUT applications, such
as 18V to 0.8V. 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. V_COMP is
defined as the output of the error amplifier. Figure 2
shows the MIC2124 control loop timing during the
steady-state operation in continuous mode. V_IL
represents the inductor current sensing voltage via the
bottom MOSFET R_DS(ON) and “Bottom Current Sense
Circuit”. When V_IL is below V_COMP, which means that the
inductor current reaches the valley value, the OFF-time
ends and ON-time is triggered. The ON-time is
predetermined by the estimation.
Figure 2. MIC2124 Control Loop Timing
Figure 3 shows the load transient operation of the
MIC2124 converter. Assume the output voltage drops
due to sudden load increase, which would cause the
inverting input of the error amplifier, which is divided
down version of V_OUT, to be slightly less than the
reference voltage, causing the output voltage of the error
amplifier V_COMP to go high. This will cause “CONTROL
LOGIC” to trigger ON-time period. At the end of the ON-
time period, a minimum OFF-time T_OFF(min) is generated
to charge BST since the inductor current V_IL is still below
V_COMP. Then, the next ON-time period is triggered due to
the high V_COMP. Therefore, the switching frequency
changes during the load transient. Also the load
regulation and transient load recovery is done by
modulating the OFF-time. With the varying duty cycle
and switching frequency, the output recovery time is fast
and the output voltage deviation is small in MIC2124
converter.
9
M9999-060810-D