MIC2128 Datasheet, PDF (17/32 Pages) Microchip Technology – 75V, Synchronous Buck Controller Featuring Adaptive On-Time Control with External Soft Start

Figure 4-2 shows operation of the MIC2128 during load
transient. The output voltage drops due to a sudden
increase in load, which results in the error amplifier
output (V_gM) falling below V_REF. This causes the
comparator to trigger an on-time event. At the end of
the ON time, a minimum OFF time t_OFF(MIN) is
generated to charge the bootstrap capacitor. The next
ON time is triggered immediately after the t_OFF(MIN) if
the error amplifier output voltage (V_gM) is still below
V_REF due to the low feedback voltage. This operation
results in higher switching frequency during load
transients. The switching frequency returns to the
nominal set frequency once the output stabilizes at new
load current level. The output recovery time is fast and
the output voltage deviation is small in the MIC2128
converter due to the varying duty cycle and switching
frequency.
Full Load
IL
No Load
VOUT
VREF
VFB
VREF
VgM
VDH
toff(MIN)
FIGURE 4-2:
Response.
MIC2128 Load Transient
Unlike true current-mode control, the MIC2128 uses
the output voltage ripple to trigger an on-time event. In
order to meet the stability requirements, the MIC2128
feedback voltage ripple should be in phase with the
inductor current ripple and large enough to be sensed
by the internal error amplifier. The recommended
feedback voltage ripple is 20 mV~100 mV over the 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 internal error amplifier. Also,
the output voltage ripple and the feedback voltage
ripple are not necessarily in phase with the inductor
 2016 Microchip Technology Inc.
MIC2128
current ripple if the ESR of the output capacitor is very
low. For these applications, ripple injection is required
to ensure proper operation. Refer to Section 5.8
“Ripple Injection” for details about the ripple injection
technique.
4.2 Soft Start (SS)
Soft Start reduces the power supply inrush current at
start-up by controlling the output voltage rise time. The
MIC2128 features SS pin which allows the user to set
the soft-start time by connecting a capacitor from the
SS pin to A_GND. An internal current source of 1.3 µA
charges this capacitor and generates a linear voltage
which is used as the reference for the internal error
amplifier during Soft Start. Once the voltage on this SS
capacitor is above the internal fixed reference of 0.6V,
the error amplifier uses the fixed 0.6V as reference
instead of the voltage on the external SS capacitor.
4.3 Current Limit (I_LIM)
The MIC2128 uses the low-side MOSFET R_DS(ON) to
sense inductor current. In each switching cycle of the
MIC2128 converter, the inductor current is sensed by
monitoring the voltage across the low-side MOSFET
during the OFF period of the switching cycle during
which low-side MOSFET is ON. An internal current
source of 96 µA generates a voltage across the
external current limit setting resistor R_CL as show in the
Figure 4-3.
VIN
MIC2128
Control
Logic
CURRENT LIMIT
DETECTION
ICL
DH
SW
DL
PGND
ILIM
L1
RCL
FIGURE 4-3:
Circuit.
MIC2128 Current Limiting
The I_LIM pin voltage (V_ILIM) is the difference of the
voltage across the low-side MOSFET and the voltage
across the resistor (V_CL). The sensed voltage V_ILIM is
compared with the power ground (P_GND) after a
blanking time of 150 ns.
If the absolute value of the voltage drop across the
low-side MOSFET is greater than the absolute value of
the voltage across the current setting resistor (V_CL), the
MIC2128 triggers the current limit event. Consecutive
DS20005620A-page 17