MIC26400_11 Datasheet, PDF (14/27 Pages) Micrel Semiconductor – 5A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 3 shows the operation of the MIC26400 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
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 in MIC26400 converter.
Figure 3. MIC26400 Load Transient Response
Unlike true current-mode control, the MIC26400 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 MIC26400 control loop has the advantage
of eliminating the need for slope compensation.
In order to meet the stability requirements, the
MIC26400 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
20mV~100mV. If a low-ESR output capacitor is selected,
the feedback voltage ripple may be too small to be
sensed by the g_m 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”
subsection in “Application Information” for more details
about the ripple injection technique.
MIC26400
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 MIC26400 implements an internal digital soft-start
by making the 0.8V reference voltage V_REF ramp from 0
to 100% in about 6ms with 9.7mV 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.
Current Limit
The MIC26400 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 MIC26400 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the inductor current is
greater than 13A, then the MIC26400 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 current-limit threshold has a foldback
characteristic related to the feedback voltage, as shown
in Figure 4.
Current-Limit Thresold
vs. Feedback Voltage
20.0
16.0
12.0
8.0
4.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
FEEDBACK VOLTAGE (V)
Figure 4. MIC26400 Current Limit Foldback Characteristic
July 2011
14
M9999-070111-C