MIC2165 Datasheet, PDF (12/27 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring Hyper Light Load

Micrel, Inc.
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 V_FB is still below V_REF. Then, the next ON-time
period is triggered due to the low V_FB. Therefore, the
switching frequency changes during the load transient.
With the varying duty-cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small in MIC2165 converter.
MIC2165
The MIC2165 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 MIC2165 automatically powers down
most of the IC circuitry and goes into a low-power mode.
Once the MIC2165 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, 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.
Figure 3. MIC2165 Load-Transient Response
Unlike in current-mode control, the MIC2165 uses the
output voltage ripple, which is proportional to the
inductor current ripple if the ESR of the output capacitor
is large enough, to trigger an ON-time period. The
predetermined ON-time makes MIC2165 control loop
have the advantage of constant on-time mode control
and eliminates the need for slope compensation.
The MIC2165 has its own stability concern: V_FB 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 V_FB ripple is
20mV~100mV. If a low ESR output capacitor is selected,
the V_FB ripple may be too small to be sensed by the gm
amplifier and the error comparator. Also, the V_OUT ripple
and the V_FB ripple are not in phase with the inductor
current ripple if the ESR of the output capacitor is very
low. Therefore, ripple injection is required for a low ESR
output capacitor. Please refer to “Ripple Injection”
subsection in “Application Information” for more details.
Discontinuous Mode
In continuous mode, the inductor current is always
greater than zero; however, at light loads the MIC2165 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 MIC2165 wakes
up and turns on the high-side MOSFET when the
feedback voltage V_FB drops below 0.8V.
Figure 4. MIC2165 Control Loop Timing
(Discontinuous Mode)
An external Schottky diode D2 is recommended in
parallel with the low-side MOSFET for high efficiency
performance as shown in the typical application
schematic. Please refer to “External Schottky Diode”
subsection in “Application Information” for more details.
During discontinuous mode, the zero crossing
comparator and the current limit comparator are turned
off. The bias current of most circuits are reduced. As a
result, the total power supply current during
discontinuous mode is only about 450μA, allowing the
MIC2165 to achieve high efficiency in light load
applications.
Soft-Start
Soft-start reduces the power supply input surge current
June 2010
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M9999-060810-D