MIC26601 Datasheet, PDF (16/30 Pages) Micrel Semiconductor – 28V, 6A Hyper Speed Control™ Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 2. MIC26601 Control Loop Timing
Figure 3 shows the operation of the MIC26601 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 MIC26601 converter.
Figure 3. MIC26601 Load Transient Response
Unlike true current-mode control, the MIC26601 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 MIC26601 control loop has the advantage
of eliminating the need for slope compensation.
MIC26601
In order to meet the stability requirements, the
MIC26601 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,
then 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.
V_DD Regulator
The MIC26601 provides a 5V regulated output for input
voltage V_IN ranging from 5.5V to 28V. When V_IN < 5.5V,
V_DD should be tied to PVIN pins to bypass the internal
linear regulator.
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 MIC26601 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 MIC26601 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 MIC26601 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor current
is greater than 13A, then the MIC26601 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.
July 2011
16
M9999-071311-A