MIC45205 Datasheet, PDF (13/31 Pages) Micrel Semiconductor – 26V/6A DC-to-DC Power Module

Micrel, Inc.
Functional Description
The MIC45205 is an adaptive on-time synchronous buck
regulator module built for high-input voltage to low-output
voltage conversion applications. The MIC45205 is
designed to operate over a wide input voltage range,
from 4.5V to 26V, and the output is adjustable with an
external resistor divider. An adaptive on-time control
scheme is employed to obtain a constant switching
frequency in steady state 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
As shown in Figure 1 (in association with Equation 1), the
output voltage is sensed by the MIC45205 feedback pin
(FB) via the voltage divider R_FB1 and R_FB2 and compared
to a 0.8V reference voltage (V_REF) at the error comparator
through a low-gain transconductance (gm) amplifier. If
the feedback voltage decreases, and the amplifier output
falls 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:
Figure 1. Output Voltage Sense via FB Pin
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.
MIC45205
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 falls 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 MIC45205 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.
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 MIC45205 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 MIC45205 during
steady state operation. The actual ON-time and resulting
switching frequency will vary with the different rising and
falling times of the 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 2 shows the MIC45205 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.
April 15, 2014
13
Revision 1.0