MIC24051 Datasheet, PDF (17/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

4.0 FUNCTIONAL DESCRIPTION
The MIC24051 is an adaptive ON-time synchronous
step-down DC/DC regulator with an internal 5V linear
regulator and a Power Good (PG) output. It is designed
to operate over a wide input voltage range from 4.5V to
19V and provides a regulated output voltage at up to 6A
of output current. An adaptive ON-time control scheme
is employed in to obtain a constant switching frequency
and to simplify the control compensation. Overcurrent
protection is implemented without the use of an
external sense resistor. The device includes an internal
soft-start function which reduces the power supply
input surge current at start-up by controlling the output
voltage rise time.
4.1 Theory of Operation
The MIC24051 operates in a continuous mode as
shown in the Block Diagram.
4.2 Continuous Mode
In continuous mode, the output voltage is sensed by
the MIC24051 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 (g_m) amplifier. If the feedback
voltage decreases and the output of the g_m amplifier is
below 0.8V, then the error comparator will trigger the
control logic and generate an ON-time period. The
ON-time period length is predetermined by the “FIXED
t_ON ESTIMATION” circuitry:
EQUATION 4-1:
Where:
t_{ONESTIMATED} = _V----_I--_N-----^V_----^O-6--^U-0---0^T---_k---_H-----_z-
V_OUT Output voltage
V_IN Power stage input voltage
At the end of the ON-time period, the internal high-side
driver turns off the high-side MOSFET and the low-side
driver turns on the low-side MOSFET. The OFF-time
period length depends upon the feedback voltage in
most cases. When the feedback voltage decreases
and the output of the gm amplifier is below 0.8V, the
ON-time period is triggered and the OFF-time period
ends. If the OFF-time period determined by the
feedback voltage is less than the minimum OFF-time
t_OFF(min), which is about 300 ns, the MIC24051 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.
MIC24051
The maximum duty cycle is obtained from the 300 ns
t_OFF(min):
EQUATION 4-2:
D_MAX
=
t_{--S----–-----}t_{--O---F----F-----M----I--N---}
t_S
=
1 – ^{3---0----0---n}---^s-
t_S
Where:
t_S 1/600 kHz = 1.66 µs
It is not recommended to use MIC24051 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 MIC24051 should be limited to 5.5V and the
maximum external ripple injection should be limited to
200 mV. Please refer to Setting Output Voltage in the
Application Information section for more details.
The actual ON-time and resulting switching frequency
will vary with the part-to-part variation in the rise and fall
times of the internal MOSFETs, the output load current,
and variations in the V_DD voltage. Also, the minimum
t_ON results in a lower switching frequency in high V_IN to
V_OUT applications, such as 18V to 1.0V. The minimum
t_ON measured on the MIC24051 evaluation board is
about 100 ns. During load transients, the switching
frequency is changed due to the varying OFF-time.
To illustrate the control loop operation, we will analyze
both the steady-state and load transient scenarios.
Figure 4-1 shows the MIC24051 control loop timing
during steady-state operation. During steady-state, the
gm amplifier senses the feedback 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 t_ON estimator.
The termination of the OFF-time is controlled by the
feedback voltage. At the valley of the feedback voltage
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.
 2016 Microchip Technology Inc.
DS20005658A-page 17