MIC2176-1 Datasheet, PDF (14/31 Pages) Micrel Semiconductor – Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
Functional Description
The MIC2176 is an adaptive on-time synchronous buck
controller family built for high-input voltage and low
output voltage applications. It is designed to operate
over a wide input voltage range from, 4.5V to 75V and
the output is adjustable with an external resistive divider.
A digitally modified adaptive on-time control scheme is
employed in to obtain a constant switching frequency
and to simplify the control compensation. Over-current
protection is implemented by sensing low-side
MOSFET’s R_DS(ON). The device features internal soft-
start, enable, UVLO, and thermal shutdown.
Theory of Operation
Figure 1 illustrates the block diagram of the MIC2176.
The output voltage is sensed by the MIC2176 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 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:
f ^{tON(estimated)}
=
V_OUT
V_IN × _SW
(1)
where V_OUT is the output voltage, V_IN is the power stage
input voltage, and f_SW is the switching frequency
(100kHz for MIC2176-1, 200kHz for MIC2176-2, and
300kHz for MIC2176-3).
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, 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
360ns, the MIC2176 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.
MIC2176
The maximum duty cycle is obtained from the 360ns
t_OFF(min):
D_max
=
t _S − t _OFF(min)
t_S
= 1− ^360ns
t_S
(2)
where t_S = 1/f_SW. It is not recommended to use MIC2176
with a OFF-time close to t_OFF(min) during steady-state
operation.
The adpative ON-time control scheme results in a
constant switching frequency in the MIC2176. 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,
such as 48V to 1.0V. The minimum t_ON measured on the
MIC2176 evaluation board is about 60ns. 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. For
easy analysis, the gain of the g_m amplifier is assumed to
be 1. With this assumption, the inverting input of the
error comparator is the same as the feedback voltage.
Figure 2 shows the MIC2176 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 plus injected
voltage 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.
Figure 2. MIC2176 Control Loop Timing
November 2010
14
M9999-111710-A