MIC24053 Datasheet, PDF (17/32 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

4.0 FUNCTIONAL DESCRIPTION
The MIC24053 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
9A of output current. It uses an adaptive ON-time
control scheme to get a constant switching frequency
and to simplify the control compensation. Overcurrent
protection is implemented without using 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 MIC24053 operates in a continuous mode, as
shown in the Package Type.
4.2 Continuous Mode
In continuous mode, the MIC24053 feedback pin (FB)
senses the output voltage through the voltage divider
(R1 and R2), and compares it 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 gm
amplifier is below 0.8V, the error comparator triggers
the control logic and generates an ON-time period. The
ON-time period length is predetermined by the “FIXED
t_ON ESTIMATION” circuitry:
EQUATION 4-1:
t_{ON estimated } = _V-----_I--_N----^V-_----^O-6--^U-0---^T_0---k---_H-----_z-
Where:
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. In most cases,
the OFF-time period length depends on the feedback
voltage. When the feedback voltage 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 the feedback voltage is
less than the minimum OFF-time (t_OFF(min)), which is
about 300 ns, the MIC24053 control logic applies 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.
MIC24053
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 using the MIC24053 with
an OFF-time close to t_OFF(min) during steady-state
operation. Also, as V_OUT increases, the internal ripple
injection increases and reduces the line regulation
performance. Therefore, the maximum output voltage
of the MIC24053 should be limited to 5.5V and the
maximum external ripple injection should be limited to
200mV. Please refer to the Setting Output Voltage
subsection in the Application Information section for
more details.
The actual ON-time and resulting switching frequency
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 MIC24053 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, analysis of both
the steady-state and load transient scenarios is
needed.
Figure 4-1 shows the MIC24053 control-loop timing
during steady-state operation. During steady-state, the
gm amplifier senses the feedback voltage ripple to
trigger the ON-time period. The feedback voltage ripple
is proportional to the output voltage ripple and the
inductor current ripple. 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.
 2015 Microchip Technology Inc.
DS20005668A-page 17