MIC24053 Datasheet, PDF (14/30 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

Micrel, Inc.
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.
Theory of Operation
The MIC24053 operates in a continuous mode, as
shown in Figure 1.
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 g_m 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:
t _{ON(estimated)}
=
V_OUT
V_IN × 600kHz
Eq. 1
where V_OUT is the output voltage and V_IN is the 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 300ns, 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 300ns
t_OFF(min):
D_max
=
t_S − t_OFF(min)
t_S
= 1− ^300ns
t_S
Eq. 2
where t_S = 1/600kHz = 1.66µs.
Micrel does not recommend 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 Application
Information 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
100ns. 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 2 shows the MIC24053 control-loop timing during
steady-state operation. During steady-state, the g_m
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.
November 2012
14
M9999-110712-A