MIC2101_13 Datasheet, PDF (21/36 Pages) Micrel Semiconductor – 38V, Synchronous Buck Controllers Featuring Adaptive On-Time Control

Micrel, Inc.
Figure 3a shows the operation of the MIC2101/02 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 MIC2101/02
converter.
Figure 3a. MIC2101/02 Load Transient Response
MIC2101/02
Discontinuous Mode (MIC2101 only)
In continuous mode, the inductor current is always
greater than zero; however, at light loads the MIC2101 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 3b. During this period, the efficiency is
optimized by shutting down all the non-essential circuits
and minimizing the supply current. The MIC2101 wakes
up and turns on the high-side MOSFET when the
feedback voltage V_FB drops below 0.8V.
The MIC2101 has a zero crossing comparator (ZC
Detection) 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 MIC2101 automatically
powers down most of the IC circuitry and goes into a
low-power mode.
Once the MIC2101 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 3b shows the control loop timing in
discontinuous mode.
Unlike true current-mode control, the MIC2101/02 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
MIC2101/02 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 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 Application
Information for more details about the ripple injection
technique.
Figure 3b. MIC2101 Control Loop Timing
(Discontinuous Mode)
During discontinuous mode, the bias current of most
circuits are reduced. As a result, the total power supply
November 13, 2013
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Revision 2.0