MIC45116 Datasheet, PDF (20/42 Pages) Microchip Technology – 20V/6A DC/DC Power Module

MIC45116
I_L
I_OUT
¨I_L(PP)
V_OUT
¨V_OUT(PP)
=
ESR
COUT
î¨I_L(PP)
V_FB
V_REF
¨V_FB(PP) = ¨V_OUT(PP) × ^RFB2
R_FB4 + R_FB2
DH
TRIGGER ON-TIME IF V_FB IS BELOW V_REF
ESTIMATED ON TIME
FIGURE 4-2:
Timing.
MIC45116 Control Loop
Figure 4-3 shows the operation of the MIC45116 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 the bootstrap capacitor (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. Note that the
instantaneous switching frequency during load
transient remains bounded and cannot increase
arbitrarily. The minimum period is limited by t_ON +
t_OFF(MIN). Because the variation in V_OUT is relatively
limited during load transient, t_ON stays virtually close to
its steady-state value.
FIGURE 4-3:
Response.
MIC45116 Load Transient
Unlike true current-mode control, the MIC45116 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
MIC45116 feedback voltage ripple should be in phase
with the inductor current ripple and is large enough to
be sensed by the gm amplifier and the error
comparator. The recommended feedback voltage
ripple is 20 mV~100 mV 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 gm 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 the Ripple Injection
subsection in the Application Information section for
more details about the ripple injection technique.
4.2 Discontinuous Mode (MIC45116-1
Only)
In continuous mode, the inductor current is always
greater than zero; however, at light loads, the
MIC45116-1 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 4-4. During this period, the
efficiency is optimized by shutting down all the
non-essential circuits and minimizing the supply
current as the switching frequency is reduced. The
MIC45116-1 wakes up and turns on the high-side
MOSFET when the feedback voltage V_FB drops below
0.8V.
The MIC45116-1 has a zero crossing comparator (Z_C)
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 MIC45116-1
automatically powers down most of the IC circuitry and
goes into a low-power mode.
Once the MIC45116-1 goes into discontinuous mode,
both low driver (DL) and high driver (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, and 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 4-4
shows the control loop timing in discontinuous mode.
DS20005571A-page 20
 2016 Microchip Technology Inc.