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Data Sheet

ADP5080

LIGHT LOAD AND OTHER MODES OF OPERATION

FOR THE DC-TO-DC CONVERTER CHANNELS

Each dc-to-dc converter channel in the ADP5080 has two or

three options to handle light load conditions, whereas asyn-

chronous dc-to-dc converters simply transition to discontinuous

conduction mode (DCM). Although light load modes provide

higher efficiency and longer battery life, they are also associated

with increased ripple and noise. This trade-off requires the user

to select the option that best suits the application, usually on a

channel by channel basis (see Table 9). The modes of operation

are illustrated in Figure 43, which shows the inductor current and

the switch node in auto PSM, auto DCM, and FPWM modes.

Slew Rate Adjustment

Each channel has a slew rate adjustment option, which is set

using the ADJ_SRx bit (where x is 1 to 6) in the OPT_SR_ADJ

node slew rate for the channel is reduced, which in turn reduces

high frequency spike noise. Enabling this feature reduces the

efficiency of the channel, however, due to increased switching

losses. For this reason, use the slew rate adjustment feature only

when low output noise is critical.

Forced PWM (FPWM) Mode

Forced pulse-width modulation (FPWM) mode maintains PWM

operation despite light load conditions, allowing negative current

to flow from the inductor through the low-side switching FET.

This mode is also referred to as continuous conduction mode

(CCM). The FPWM option has the lowest efficiency, but may

be selected when constant frequency and low ripple are absolutely

required, regardless of load.

Auto DCM

Automatic discontinuous conduction mode (auto DCM) is

available on Channel 5 and Channel 6. Auto DCM turns off the

low-side switching FET when the inductor current falls to zero

during the tOFF period, preventing negative current from flowing

through the low-side FET. This operation is equivalent to that of

traditional flywheel diode-based PWM regulators. Auto DCM has

higher efficiency than FPWM mode because negative inductor

current is not allowed, but rather is recirculated to the input side.

At very light loads in auto DCM, some pulse skipping occurs and,

therefore, switching is not at a constant frequency.

Auto PSM

Automatic power save mode (auto PSM) is similar to auto DCM,

except that it intentionally turns on the high-side FET with a fixed

period (approximately 80% of nominal tON). This operation forces

the regulator to skip a number of PWM cycles. Compared to auto

DCM, auto PSM skips a larger number of cycles and begins skip-

ping cycles at a higher load current. Auto PSM reduces switching

losses dramatically and improves efficiency, as shown in Figure 44.

However, in light load conditions, larger output voltage ripple

can be expected.

Figure 43. Auto PSM, Auto DCM, and FPWM Operation (Switch Node and

Inductor Current Shown, Dashed Line Indicates 0 A)

Figure 44. Efficiency of Auto PSM, Auto DCM, and FPWM Operation

Selecting Light Load Switching Modes

Each dc-to-dc converter channel can be configured with its

own light load switching mode using the AUTO-PSMx bits

in Register 28 and, for Channel 5 and Channel 6, the DCM56

bit in Register 32 (see Table 10 and Table 11).

Table 10. Light Load Switching Modes, Channel 1 to

Channel 4

AUTO-PSMx Bit

Light Load Switching Mode

FPWM

Auto PSM

Table 11. Light Load Switching Modes, Channel 5 and

Channel 6

AUTO-PSMx Bit

DCM56 Bit

Light Load Switching Mode

FPWM

Auto PSM

Auto DCM

X = don’t care.

AUTO PSM

AUTO DCM

FPWM

100

OUTPUT CURRENT (A)

0.01

0.1

AUTO PSM

AUTO DCM

FPWM

Rev. A | Page 27 of 64

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