MIC26903ZA_14 Datasheet, PDF (16/29 Pages) Micrel Semiconductor – 28V, 9A Hyper Speed Control Synchronous DC/DC Buck Regulator

Micrel, Inc.
Figure 1. MIC26903-ZA Control Loop Timing
Figure 2 shows the operation of the MIC26903-ZA during
a load transient. The output voltage drops because of the
sudden load increase, which makes the V_FB 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 by 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. With the varying duty cycle and switching
frequency, the output recovery time is fast and the output
voltage deviation is small in MIC26903-ZA converter.
Figure 2. MIC26903-ZA Load Transient Response
MIC26903-ZA
Unlike true current-mode control, the MIC26903-ZA 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 MIC26903-ZA control loop has the advantage
of eliminating the need for slope compensation.
To meet the stability requirements, the MIC26903-ZA
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 the “Ripple Injection”
subsection in Application Information for more details
about the ripple injection technique.
VDD Regulator
The MIC26903-ZA provides a 5V regulated output for
input voltage VIN ranging from 5.5V to 28V. When
VIN < 5.5V, VDD 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 MIC26903-ZA implements an internal digital soft-
start by making the 0.6V reference voltage V_REF ramp
from 0 to 100% in about 6ms in 9.7mV steps. Therefore,
the output voltage is controlled to increase slowly by a
stair-case V_FB ramp. After the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption. VDD must be powered up at the same time
or after VIN to make the soft-start function correctly.
Current Limit
The MIC26903-ZA uses the R_DS(ON) of the internal low-
side power MOSFET to sense overcurrent conditions.
This method avoids 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.
July 22, 2014
16
Revision 1.1