MIC28513 Datasheet, PDF (18/34 Pages) Microchip Technology – 45V, 4A Synchronous Buck Regulator

MIC28513
I_L
I_OUT
¨I_L(PP)
V_OUT
¨V_OUT(PP) = ESR_COUT× ¨I_L(PP)
V_FB
V_REF
¨V_FB(PP)
=
¨V_OUT(PP)
×
R2
R1 + R2
HSD _{TRIGGER ON-TIME IF VFB IS BELOW VREF}
ESTIMATED ON-TIME
FIGURE 4-2:
Timing.
MIC28513 Control Loop
Figure 4-3 shows the operation of the MIC28513 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 because 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 MIC28513 converter.
I_OUT
NO LOAD
V_OUT
FULL LOAD
V_FB
HSD
V_REF
FIGURE 4-3:
Response.
T_OFF(MIN)
MIC28513 Load Transient
Unlike true current-mode control, the MIC28513 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 MIC28513 control loop has the advantage
of eliminating the need for slope compensation.
In order to meet the stability requirements, the
MIC28513 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 20 mV ~
100 mV.
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, if
the ESR of the output capacitor is very low, the output
voltage ripple and the feedback voltage ripple are not
necessarily in phase with the inductor current ripple. In
these cases, ripple injection is required to ensure
proper operation. Please refer to the Ripple Injection
subsection for more details about the ripple injection
technique.
4.2 Discontinuous Mode (MIC28513-1
Only)
In continuous mode, the inductor current is always
greater than zero; however, at light loads the
MIC28513-1 is able to force the inductor current to
operate in discontinuous mode. Discontinuous mode
occurs when 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. The MIC28513-1 wakes up and turns on the
high-side MOSFET when the feedback voltage V_FB
drops below 0.8V.
The MIC28513-1 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 MIC28513-1 automatically powers
down most of the IC circuitry and goes into a low-power
mode.
Once the MIC28513-1 goes into discontinuous mode,
both DH and DL 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.
DS20005522A-page 18
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