MIC2165_1011 Datasheet, PDF (12/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
Figure 2. MIC2165 Control Loop Timing
(Continuous Mode)
Figure 3 shows the load transient operation of the
MIC2165 converter. The output voltage drops due to the
sudden load increase, which causes 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 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.
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 g_m 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.
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.
September 2010
12
M9999-092410-E