MIC28510_12 Datasheet, PDF (16/30 Pages) Micrel Semiconductor – 75V/4A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 3 shows the operation of the MIC28510 during
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 MIC28510
converter.
Figure 3. MIC28510 Load Transient Response
Unlike true current–mode control, the MIC28510 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.
In order to meet the stability requirements, the
MIC28510 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 of
this datatsheet for more details regarding the ripple
injection technique.
MIC28510
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 MIC28510 implements an internal digital soft–start
by making the 0.8V reference voltage V_REF ramp from 0
to 100% in approximately 6ms with 9.7mV steps.
Therefore, the output voltage is controlled to increase
slowly with 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 allow the soft–start function
correctly.
Current Limit
The MIC28510 uses the R_DS(ON) of the internal low–side
power MOSFET to sense over–current conditions. This
method will avoid adding cost, use of additional board
space and power losses taken by a discrete current
sense resistor.
In each switching cycle of the MIC28510 converter, the
inductor current is sensed by monitoring the low–side
MOSFET in the OFF period. If the peak inductor current
is greater than 7A, then the MIC28510 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 Threshold
vs. Feedback Voltage
12
V_IN = 48V
10
8
6
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
FEEDBACK VOLTAGE (V)
Figure 4. MIC28510 Current–Limit Foldback Characteristic
March 2012
16
M9999-030912-A