MIC24053 Datasheet, PDF (18/32 Pages) Micrel Semiconductor – 12V, 9A High-Efficiency Buck Regulator

MIC24053
I_L
I_OUT
V_OUT
V_FB
V_REF
HSD
ΔI_L(pp)
ΔV_OUT(pp) = ESR_Cout * ΔI_L(pp)
ΔV_FB(pp) = ΔV_OUT(pp) *
R2
R1+R2
Trigger ON-time if V_FB is below V_REF
Estimated ON-Time
FIGURE 4-1:
Timing.
MIC24053 Control Loop
Figure 4-2 shows the operation of the MIC24053 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 causes 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 because of
the low feedback voltage. Therefore, the switching
frequency changes during the load transient, but
returns to the nominal fixed frequency after the output
has stabilized at the new load current level. Because of
the varying duty cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small in the MIC24053 converter.
I_OUT
No load
V_OUT
Full load
V_FB
HSD
V_REF
FIGURE 4-2:
Response.
T_OFF(min)
MIC24053 Load Transient
Unlike true current-mode control, the MIC24053 uses
the output voltage ripple to trigger an ON-time period.
The output voltage ripple is proportional to the inductor
DS20005668A-page 18
current ripple if the ESR of the output capacitor is large
enough. The MIC24053 control loop has the advantage
of eliminating the need for slope compensation.
To meet the stability requirements, the MIC24053
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 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 Application Information
for more details about the ripple injection technique.
4.3 V_DD Regulator
The MIC24053 provides a 5V regulated output for input
voltage V_IN ranging from 5.5V to 19V. When
VIN < 5.5V, tie V_DD to the PV_IN pins to bypass the
internal linear regulator.
4.4 Soft-Start
Soft-start reduces the power supply input surge current
at start-up by controlling the output voltage rise time.
The input surge appears while the output capacitor is
charged up. A slower output rise time draws a lower
input surge current.
The MIC24053 implements an internal digital soft-start
by making the 0.8V reference voltage (V_REF) ramp
from 0 to 100% in about 3 ms with 9.7 mV steps.
Therefore, the output voltage is controlled to increase
slowly by a staircase V_FB ramp. After the soft-start
cycle ends, the related circuitry is disabled to reduce
current consumption. V_DD must be powered up at the
same time or after V_IN to make the soft-start function
correctly.
4.5 Current Limit
The MIC24053 uses the R_DS(ON) of the internal
low-side power MOSFET to sense overcurrent
conditions. This method reduces cost, board space,
and power losses taken by a discrete current sense
resistor. The low-side MOSFET is used because it
displays much lower parasitic oscillations during
switching than the high-side MOSFET.
In each switching cycle of the MIC24053 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor
current is greater than 14A, the MIC24053 turns off the
high-side MOSFET and a soft-start sequence is
triggered. This mode of operation is called “hiccup
mode.” Its purpose is to protect the downstream load in
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