MIC24051 Datasheet, PDF (18/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

MIC24051
FIGURE 4-1:
Timing.
MIC24051 Control Loop
Figure 4-2 shows the operation of the MIC24051 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 MIC24051 converter.
FIGURE 4-2:
Response.
MIC24051 Load Transient
Unlike true current-mode control, the MIC24051 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 MIC24051 control loop has the advantage
of eliminating the need for slope compensation.
In order to meet the stability requirements, the
MIC24051 feedback voltage ripple should be in phase
with the inductor current ripple and large enough to be
DS20005658A-page 18
sensed by the gm 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
Ripple Injection in the Application Information section
for more details about the ripple injection technique.
4.3 V_DD Regulator
The MIC24051 provides a 5V regulated output for input
voltage V_IN ranging from 5.5V to 19V. When V_IN < 5.5V,
V_DD should be tied to PV_IN pins to bypass the internal
linear regulator.
4.4 Soft-Start
Soft-start reduces the power supply input surge current
at startup by controlling the output voltage rise time.
The input surge appears while the output capacitor is
charged up. A slower output rise time will draw a lower
input surge current.
The MIC24051 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
stair-case V_FB ramp. Once 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 MIC24051 uses the R_DS(ON) of the internal
low-side power MOSFET to sense over-current
conditions. This method will avoid adding 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 MIC24051 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor
current is greater than 11A, then the MIC24051 turns
off the high-side MOSFET and a soft-start sequence is
triggered. This mode of operation is called “hiccup
mode” and its purpose is to protect the downstream
load in case of a hard short. The load current-limit
threshold has a fold-back characteristic related to the
feedback voltage as shown in Figure 4-3.
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