MIC28511 Datasheet, PDF (18/34 Pages) Microchip Technology – 60VIN, 3A Synchronous Buck Regulator

MIC28511
4.0 FUNCTIONAL DESCRIPTION
The MIC28511 is an adaptive on-time synchronous
buck regulator with integrated high-side and low-side
MOSFETs suitable for high-input voltage to low-output
voltage conversion applications. It is designed to
operate over a wide input voltage range (4.6V to 60V)
which is suitable for automotive and industrial
applications. The output is adjustable with an external
resistive divider. An adaptive on-time control scheme is
employed to produce a constant switching frequency in
continuous-conduction mode and reduced switching
frequency in discontinuous-operation mode, improving
light-load efficiency. Overcurrent protection is
implemented by sensing low-side MOSFET’s R_DS(ON).
The device features internal soft-start, enable, UVLO,
and thermal shutdown.
4.1 Theory of Operation
As illustrated in the Functional Block Diagram, the
output voltage of the MIC28511 is sensed by the
feedback (FB) pin via voltage dividers 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
amplifier output 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 estimator circuitry:
EQUATION 4-1:
Where:
t_{ONESTIMATED}
=
-----^V----^O----^U----^T------
V_IN  f_SW
V_OUT
V_IN
f_SW
Output Voltage
Power Stage Input Voltage
Switching Frequency
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 g_M amplifier is below 0.8V, then
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 200 ns (typical), the MIC28511
control logic will apply the t_OFF(MIN) instead. The
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
Equation 4-2.
EQUATION 4-2:
D_MAX = 1 – t_{OFFMIN}  f_SW
It is not recommended to use MIC28511 with an
OFF-time close to t_OFF(MIN) during steady-state
operation.
The adaptive ON-time control scheme results in a
constant switching frequency in the MIC28511. The
actual ON-time and resulting switching frequency will
vary with the different rising and falling times of the
external MOSFETs. Also, the minimum t_ON results in a
lower switching frequency in high V_IN to V_OUT
applications. During load transients, the switching
frequency is changed due to the varying OFF-time.
Figure 4-1 shows the allowable range of the output
voltage versus the input voltage. The minimum output
voltage is 0.8V which is limited by the reference
voltage. The maximum output voltage is 24V which is
limited by the internal circuitry.
p
g
30
25
20
f_SW = 600kHz
f_SW = 400kHz
15
f_SW = 200kHz
10
ALLOWABLE RANGE
5
0.8V (MINIMUM)
0
5 15 25 35 45 55
INPUT VOLTAGE (V)
FIGURE 4-1:
Allowable Output Voltage
Range vs. Input Voltage.
To illustrate the control loop operation, both the
steady-state and load transient scenarios will be
analyzed.
Figure 4-2 shows the MIC28511 control loop timing
during steady-state operation. During steady-state, the
g_M 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.
DS20005520A-page 18
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