MIC261203 MICREL [Micrel Semiconductor], MIC261203 Datasheet - Page 16

• Manufacturer Part Number: MIC261203
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Figure 3 shows the operation of the MIC261203 during a
load transient. The output voltage drops due to the
sudden load increase, which causes the V
than V
an ON-time period. At the end of the ON-time period, a
minimum OFF-time t
since the feedback voltage is still below V
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 MIC261203
converter.
July 2011
Figure 3. MIC261203 Load Transient Response
REF
Figure 2. MIC261203 Control Loop Timing
. This will cause the error comparator to trigger
OFF(min)
is generated to charge C
REF
FB
. Then, the
to be less
BST
16
Unlike true current-mode control, the MIC261203 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 MIC261203 control loop has the advantage
of eliminating the need for slope compensation.
In order to meet the stability requirements, the
MIC261203 feedback voltage ripple should be in phase
with the inductor current ripple and large enough to be
sensed by the g
The
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
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
MIC261203 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
efficiency is optimized by shutting down all the non-
essential circuits and minimizing the supply current. The
MIC261203 wakes up and turns on the high-side
MOSFET when the feedback voltage V
0.8V.
The MIC261203 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
negative, then the MIC261203 automatically powers
down most of the IC circuitry and goes into a low-power
mode.
Once the MIC261203 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
causes V
wake up into normal continuous mode. First, the bias
currents
discontinuous mode are restored, then a t
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.
FB
recommended
L
) shown in Figure 4. During this period, the
> 0.8V and the inductor current goes slightly
FB
of
to go below V
most
m
m
amplifier and the error comparator.
amplifier and the error comparator.
circuits
feedback
OUT
REF
, then all the circuits will
drops. If the drop of V
reduced
voltage
M9999-071311-A
FB
MIC261203
drops below
during
ON
ripple
pulse is
OUT
the
is