ADP3208 ON Semiconductor, ADP3208 Datasheet - Page 22

• Manufacturer Part Number: ADP3208
• Description: 7-bit, Programmable, Dual-phase, Mobile, Cpu, Synchronous Buck Controller
• Manufacturer: ON Semiconductor

ADP3208
Light Load RPM DCM Operation
In single-phase normal mode, DPRSLP is pulled low and the
APD3208 operates in continuous conduction mode (CCM)
over the entire load range. The upper and lower MOSFETs run
synchronously and in complementary phase. See Figure 29 for
the typical waveforms of the ADP3208 running in CCM with a
7 A load current.
If DPRSLP is pulled high, the ADP3208 operates in RPM mode.
If the load condition is light, the chip enters discontinuous con-
duction mode (DCM). Figure 30 shows a typical single-phase
buck with one upper FET, one lower FET, an output inductor,
an output capacitor, and a load resistor. Figure 31 shows the
path of the inductor current with the upper FET on and the
lower FET off. In Figure 32 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 33). Figure 34
shows the inductor current and switch node voltage in DCM.
In DCM with a light load, the ADP3208 monitors the switch node
voltage to determine when to turn off the low-side FET. Figure 35
shows a typical waveform in DCM with a 1 A load current. Between
t
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 t
voltage is approximately −6 mV, the low-side FET is turned off.
Figure 34 shows a small, dampened ringing at t
the LC created from capacitance on the switch node, including
the C
The ADP3208 automatically goes into DCM with a light load.
Figure 35 shows the typical DCM waveform of the ADP3208.
As the load increases, the ADP3208 enters into CCM. In DCM,
frequency decreases with load current. Figure 36 shows switching
frequency vs. load current for a typical design. In DCM, switching
1
and t
DS
4
2
3
1
2
of the FETs and the output inductor. This ringing is normal.
, the inductor current ramps down. The current flows
Figure 29. Single-Phase Waveforms in CCM
SWITCH NODE 5V/DIV
LOW-SIDE GATE DRIVE 5V/DIV
INDUCTOR CURRENT 5A/DIV
OUTPUT VOLTAGE 20mV/DIV
400ns/DIV
2
. When the switch
2
. This is caused by
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frequency is a function of the inductor, load current, input voltage,
and output voltage.
INDUCTOR
CURRENT
VOLTAGE
VOLTAGE
SWITCH
NODE
INPUT
Figure 31. Buck Topology Inductor Current During t
Figure 32. Buck Topology Inductor Current During t
Figure 33. Buck Topology Inductor Current During t
Figure 34. Inductor Current and Switch Node in DCM
t
0
t
1
DRVH
DRVL
Figure 30. Buck Topology
OFF
OFF
OFF
OFF
ON
ON
Q1
Q2
t
2
SWITCH
NODE
L
L
L
L
t
3
t
4
C
C
C
C
VOLTAGE
OUTPUT
LOAD
LOAD
LOAD
0
1
2
and t
and t
and t
LOAD
1
2
3