ADP2121 Analog Devices, ADP2121 Datasheet - Page 15

• Manufacturer Part Number: ADP2121
• Description: 600 mA, 6 MHz Synchronous Step-Down DC-to-DC Converter
• Manufacturer: Analog Devices

INDUCTOR CURRENT (mA)
Auto Mode (PFM and PWM Switching)
Auto mode is a power-saving feature that enables the converter
to switch between PWM and PFM in response to the output
load. Auto mode is enabled when the MODE pin is pulled low.
In auto mode, the ADP2121 operates in PFM mode for light load
currents and switches to PWM mode for medium and heavy load
currents. Figure 53 uses the typical threshold values of the 1.82 V
output voltage option to demonstrate the behavior of the ADP2121
in auto mode. The threshold values will shift accordingly for other
output voltages.
Pulse Frequency Modulation (PFM)
When the converter is operating under light load conditions,
the effective switching frequency and supply current are
decreased and varied using PFM to regulate the output voltage.
This results in improved efficiencies and lower quiescent
currents. In PFM mode, the converter only switches when
necessary to keep the output voltage within the PFM limits set
by an internal comparator (see Figure 53). Switching stops
when the upper limit is reached and resumes when the lower
limit is reached.
When the upper level is reached, the output stage and oscillator
turn off to reduce the quiescent current. During this stage, the
output capacitor supplies the current to the load. As the output
capacitor discharges and the output voltage reaches the lower
PFM comparator threshold, switching resumes and the process
OUTPUT VOLTAGE (V)
*PFM AND PWM THRESHOLD VARIES WITH INPUT VOLTAGE.
SEE FIGURE 22, FIGURE 23 and FIGURE 24 FOR TYPICAL VALUES.
Figure 53. PFM-to-PWM Transition Point, V
OUTPUT VOLTAGE (20mV/DIV, 1.82V DC OFFSET)
1.875V
1.820V
INDUCTOR CURRENT (200mA/DIV)
Figure 52. Typical PWM Operation
SWITCH NODE VOLTAGE (1V/DIV)
0mA
TIME (100ns/DIV)
I
OUT
MODE TRANSITION POINT*
= 70mA TO 170mA (TYPICAL)
V
V
I
OUT
IN
OUT
OUT
= 3.6V
= 200mA
= 1.82V
= 1.82 V
TIME (µs)
TIME (µs)
Rev. B | Page 15 of 20
repeats. The output voltage, switching node voltage, and
inductor current during this process are shown in Figure 54.
Mode Transition
When the MODE pin is low, the converter switches between
PFM and PWM modes automatically to maintain optimal
transient response and efficiency. The mode transition point
depends on the input voltage. Hysteresis exists in the transition
point to prevent instability and decreased efficiencies that could
result if the converter were able to oscillate between PFM and
PWM for a fixed input voltage and load current. See Figure 22,
Figure 23, and Figure 24 for typical values.
A switch from PFM to PWM occurs when the output voltage dips
below the nominal value of the output voltage option. Switching
to PWM allows the converter to maintain efficiency and supply
a larger current to the load.
The switch from PWM to PFM occurs when the output current
is below the PFM threshold for multiple consecutive switching
cycles. Switching to PFM allows the converter to save power by
supplying the lighter load current with fewer switching cycles.
Figure 53 shows that the output voltage in PFM mode is slightly
higher to keep the ADP2121 from oscillating between modes,
ensuring stable operation.
ENABLE/SHUTDOWN
The EN input turns the ADP2121 on or off. Connect EN to
GND or logic low to shut down the part and reduce the current
consumption to 1.0 µA (maximum). Connect EN to VIN or to
logic high to enable the part. Do not leave this pin floating.
INTERNAL CONTROL FEATURES
Overcurrent Protection
To ensure that excessively high currents do not damage the
inductor, the ADP2121 incorporates cycle-by-cycle overcurrent
protection. This function is accomplished by monitoring the
instantaneous peak current on the power PMOS switch. If this
current exceeds the maximum level (1 A typical), the PMOS is
immediately turned off. This minimizes the potential for damage
to power components during certain faults and transient events.
The value listed in Table 2 is an open loop dc tested value. Inherent
OUTPUT VOLTAGE (20mV/DIV, 1.82V DC OFFSET)
Figure 54. Typical PFM Operation
SWITCH NODE VOLTAGE
INDUCTOR CURRENT
(200mA/DIV)
TIME (1µs/DIV)
(2V/DIV)
V
V
I
OUT
IN
OUT
= 3.6V
= 25mA
= 1.82V
ADP2121