MIC28304 Datasheet, PDF (22/39 Pages) Micrel Semiconductor – 70V 3A Power Module

Micrel, Inc.
Functional Description
The MIC28304 is an adaptive on-time synchronous buck
regulator module built for high-input voltage to low-output
voltage conversion applications. The MIC28304 is
designed to operate over a wide input voltage range,
from 4.5V to 70V, and the output is adjustable with an
external resistor divider. An adaptive on-time control
scheme is employed to obtain a constant switching
frequency and to simplify the control compensation.
Hiccup mode 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.
The module has integrated switching FETs, inductor,
bootstrap diode, resistor and capacitor.
Theory of Operation
Per the Functional Diagram of the MIC28304 module, the
output voltage is sensed by the MIC28304 feedback pin
FB via the voltage divider R1 and R11, and compared to
a 0.8V reference voltage VREF at the error comparator
through a low-gain transconductance (gm) 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 tON
Estimator” circuitry:
t _{ON(ESTIMATED)}
=
V_OUT
V_IN × f_SW
Eq. 1
where V_OUT is the output voltage, V_IN is the power stage
input voltage, and f_SW is the 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, 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
200ns, the MIC28304 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.
MIC28304
The maximum duty cycle is obtained from the 200ns
t_OFF(MIN):
D_MAX
=
t_S
− t_OFF(MIN)
t_S
= 1−
200ns
t_S
Eq. 2
Where:
t_S = 1/f_SW. It is not recommended to use MIC28304 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 MIC28304. 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.
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 1 shows the MIC28304 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.
March 25, 2014
22
Revision 1.1