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ADP3208D Datasheet(PDF) 21 Page - ON Semiconductor

Part No. ADP3208D
Description  7-Bit, Programmable, Dual-Phase, Mobile, CPU, Synchronous Buck Controller
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADP3208D Datasheet(HTML) 21 Page - ON Semiconductor

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ADP3208D
http://onsemi.com
21
If DPRSLP is pulled high, the ADP3208D operates in
RPM mode. If the load condition is light, the chip enters
Discontinuous Conduction Mode (DCM). Figure 33 shows
a typical single−phase buck with one upper FET, one lower
FET, an output inductor, an output capacitor, and a load
resistor. Figure 34 shows the path of the inductor current
with the upper FET on and the lower FET off. In Figure 35
the high−side FET is off and the low−side FET is on. In
CCM, if one FET is on, its complementary FET must be off;
however, in DCM, both high− and low−side FETs are off and
no current flows into the inductor (see Figure 36). Figure 37
shows the inductor current and switch node voltage in DCM.
In DCM with a light load, the ADP3208D monitors the
switch node voltage to determine when to turn off the
low−side FET. Figure 38 shows a typical waveform in DCM
with a 1 A load current. Between t1 and t2, the inductor current
ramps down. The current flows through the source drain of
the low−side FET and creates a voltage drop across the FET
with a slightly negative switch node. As the inductor current
ramps down to 0 A, the switch voltage approaches 0 V, as seen
just before t2. When the switch voltage is approximately
−6 mV, the low−side FET is turned off.
Figure 37 shows a small, dampened ringing at t2. This is
caused by the LC created from capacitance on the switch
node, including the CDS of the FETs and the output inductor.
This ringing is normal.
The ADP3208D automatically goes into DCM with a light
load. Figure 38 shows the typical DCM waveform of the
ADP3208D. As the load increases, the ADP3208D enters
into CCM. In DCM, frequency decreases with load current.
Figure 39 shows switching frequency vs. load current for a
typical design. In DCM, switching frequency is a function
of the inductor, load current, input voltage, and output
voltage.
Figure 33. Buck Topology
SWITCH
NODE
L
DRVL
DRVH
Q1
Q2
C
OUTPUT
VOLTAGE
LOAD
INPUT
VOLTAGE
Figure 34. Buck Topology Inductor Current During
t0 and t1
L
C
ON
OFF
LOAD
Figure 35. Buck Topology Inductor Current During
t1 and t2
L
C
ON
OFF
LOAD
Figure 36. Buck Topology Inductor Current During
t2 and t3
L
C
OFF
OFF
LOAD
Figure 37. Inductor Current and Switch Node in DCM
INDUCTOR
CURRENT
SWITCH
NODE
VOLTAGE
t0 t1
t2
t3 t4
Figure 38. Single−Phase Waveforms in DCM with 1 A
Load Current
3
1
2
4
2 ms/DIV
SWITCH NODE 5.0 V/DIV
LOW−SIDE GATE DRIVE 5.0 V/DIV
OUTPUT VOLTAGE
20 mV/DIV
INDUCTOR CURRENT
5 A/DIV


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