UM3506QA Datasheet, PDF (6/9 Pages) Union Semiconductor, Inc. – Synchronous Step-Down Converter

Block Diagram
UM3506QA
VIN
REF
0.6V
OSC
SLOPE
COMP
NC
2,5,
8
I SENSE
AMP
0.6V
+
EA
FB 4
-
-
ICOMP
+
_
SQ
__
RQ
RS LATCH
PWM
LOGIC
DRV
EN 9
MODE 3
I ZERO
COMP
UM3506QA
L1
L2
6,7
13,
14
10 VIN
1 SW
11,
12
GND
Function Description
Integrated Inductor
The UM3506QA utilizes a low loss, multilayer inductor. The DCR of the integrated inductor is
180mΩ and the inductor is about 2.2µH. The use of an internal inductor localizes the noise
associated with the output loop currents. The proprietary integrated inductor construction reduces
the area of the converter’s large current loop that can reduce the radiated noise coupled into the
traces of the circuit board. Furthermore, the package layout is optimized to reduce the electrical
path length for the AC ripple currents that are a major source of radiated emissions from DC-DC
converters. The integrated inductor significantly reduces parasitic effects that can harm loop
stability, and makes layout very simple. All these lead to lower output noise and fewer influences
on the input power.
Current Mode PWM Control and Current Limit
The UM3506QA uses constant frequency, current mode step-down architecture. Both the main
(P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. From the
block diagram, a comparator ICOMP is used to realize current limit protection. Lossless current
sensing converts the peak current signal to a voltage to sum in with the internal slope
compensation. This summed signal is compared to the error amplifier output to provide a peak
current control command for the PWM. The cycle-by-cycle current limit is set at 1200mA
(typical). During normal operation, the internal top power MOSFET is turned on each cycle when
the oscillator sets the RS latch, and turned off when the current comparator ICOMP, resets the RS
latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output
of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback
voltage, FB, relative to the 0.6V reference, which in turn, causes the EA amplifier’s output
voltage to increase until the average inductor current matches the new load current. While the top
MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to
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http://www.union-ic.com Rev.01 Oct.2016
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