MIC26603 Datasheet, PDF (16/30 Pages) Micrel Semiconductor – 28V, 6A Hyper Light Load Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 2. MIC26603 Control Loop Timing
Figure 3 shows the operation of the MIC26603 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 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 in MIC26603 converter.
Figure 3. MIC26603 Load Transient Response
MIC26603
Unlike true current-mode control, the MIC26603 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. The MIC26603 control loop has the advantage
of eliminating the need for slope compensation.
In order to meet the stability requirements, the
MIC26603 feedback voltage ripple should be in phase
with the inductor current ripple and large enough to be
sensed by the g_m amplifier and the error comparator.
The recommended feedback voltage ripple is
20mV~100mV. 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 Application Information for more details
about the ripple injection technique.
Discontinuous Mode
In continuous mode, the inductor current is always
greater than zero; however, at light loads the MIC26603
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. The MIC26603 wakes
up and turns on the high-side MOSFET when the
feedback voltage V_FB drops below 0.8V.
The MIC26603 has a zero crossing comparator 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 MIC26603 automatically powers down
most of the IC circuitry and goes into a low-power mode.
Once the MIC26603 goes into discontinuous mode, both
LSD and HSD 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, 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.
July 2011
16
M9999-071311-A