MIC45205 Datasheet, PDF (14/31 Pages) Micrel Semiconductor – 26V/6A DC-to-DC Power Module

Micrel, Inc.
Figure 2. MIC45205 Control Loop Timing
Figure 3 shows the operation of the MIC45205 during a
load transient. The output voltage drops due to the
sudden load increase, which causes the V_FB to be less
than V_REF. This will cause the error comparator to trigger
an ON-time period. At the end of the ON-time period, a
minimum OFF-time t_OFF(MIN) is generated to charge the
bootstrap capacitor (C_BST) since the feedback voltage is
still below V_REF. Then, the next ON-time period is
triggered due to the low feedback voltage. Therefore, the
switching frequency changes during the load transient,
but returns to the nominal fixed frequency once the
output has stabilized at the new load current level. With
the varying duty cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small. Note that the instantaneous switching
frequency during load transient remains bounded and
cannot increase arbitrarily. The minimum is limited by t_ON
+ t_OFF(MIN) .Since the variation in V_OUT is relatively limited
during load transient, t_ON stays virtually close to its
steady-state value.
Figure 3. MIC45205 Load Transient Response
MIC45205
Unlike true current-mode control, the MIC45205 uses the
output voltage ripple to trigger an ON-time period. The
output voltage ripple is proportional to the inductor
current ripple if the ESR of the output capacitor is large
enough.
In order to meet the stability requirements, the MIC45205
feedback voltage ripple should be in phase with the
inductor current ripple and are large enough to be sensed
by the g_m amplifier and the error comparator. The
recommended feedback voltage ripple is 20mV~100mV
over full input voltage range. If a low ESR output
capacitor is selected, then the feedback voltage ripple
may be too small to be sensed by the g_m amplifier and
the error comparator. Also, the output voltage ripple and
the feedback voltage ripple are not necessarily in phase
with the inductor current ripple if the ESR of the output
capacitor is very low. In these cases, ripple injection is
required to ensure proper operation. Please refer to
“Ripple Injection” subsection in the Application
Information section for more details about the ripple
injection technique.
Discontinuous Mode (MIC45205-1 only)
In continuous mode, the inductor current is always
greater than zero; however, at light loads, the MIC45205-
1 is able to force the inductor current to operate in
discontinuous mode. Discontinuous mode is where the
inductor current falls to zero, as indicated by trace (I_L)
shown in Figure 4. During this period, the efficiency is
optimized by shutting down all the non-essential circuits
and minimizing the supply current as the switching
frequency is reduced. The MIC45205-1 wakes up and
turns on the high-side MOSFET when the feedback
voltage V_FB drops below 0.8V.
The MIC45205-1 has a zero crossing comparator (ZC)
that monitors the inductor current by sensing the voltage
drop across the low-side MOSFET during its ON-time. If
the V_FB > 0.8V and the inductor current goes slightly
negative, then the MIC45205-1 automatically powers
down most of the IC circuitry and goes into a low-power
mode.
Once the MIC45205-1 goes into discontinuous mode,
both DL and DH are low, which turns off the high-side
and low-side MOSFETs. The load current is supplied by
the output capacitors and V_OUT drops. If the drop of V_OUT
causes V_FB to go below V_REF, then all the circuits will
wake up into normal continuous mode. First, the bias
currents of most circuits reduced during the
discontinuous mode are restored, and then a t_ON pulse is
triggered before the drivers are turned on to avoid any
possible glitches. Finally, the high-side driver is turned
on. Figure 4 shows the control loop timing in
discontinuous mode.
April 15, 2014
14
Revision 1.0