Data Sheet

ADP5135

Rev. 0 | Page 15 of 24

Figure 36. Regulator Sequencing on the ADP5135, Showing Precision Enable (PE) Thresholds, with Power Good

BUCK REGULATORS: BUCK1, BUCK2, AND BUCK3

The buck regulators use a fixed frequency and high speed

current mode architecture. The buck operates with an input

voltage of 3.0 V to 5.5 V.

The buck regulator output voltage is resistor programmable

from 0.8 V up to 3.8 V, shown in Figure 37 for BUCK1. The

ratio of R1 and R2 multiplied by the feedback voltage

determines the voltage level at the output. If, for example, R1

and R2 have equal resistance values, the output voltage is set to

1.0 V. The output voltage can optionally be factory programmed

to default values, as indicated in the Ordering Guide section. In

this event, R1 and R2 are not needed, and FB1 can be left

unconnected. In all cases, VOUT1 must be connected to the

output capacitor. FB1 is 0.5 V.

Figure 37. BUCK1 External Output Voltage Setting

Control Scheme

The buck regulators operate with a fixed frequency, current

mode PWM control architecture at medium to high loads for

high efficiency but shift to a power save mode (PSM) control

scheme at light loads to lower the regulation power losses.

When operating in fixed frequency PWM mode, the duty cycle

of the integrated switches is adjusted and regulates the output

voltage. When operating in PSM at light loads, the output

voltage is controlled in a hysteretic manner, with higher output

voltage ripple. During this time, the converter is able to stop

switching and enters an idle mode, which improves conversion

efficiency.

PWM Mode

In PWM mode, the bucks operate at a fixed frequency of 3 MHz

set by an internal oscillator. At the start of each oscillator cycle,

the PFET switch turns on, sending a positive voltage across the

inductor. Current in the inductor increases until the current

sense signal crosses the peak inductor current threshold that

turns off the PFET switch and turns on the NFET synchronous

rectifier, sending a negative voltage across the inductor and

causing the inductor current to decrease. The synchronous

rectifier stays on for the rest of the cycle. The buck regulators

regulate the output voltage by adjusting the peak inductor

current threshold.

4

2

0

4

2

0

4

2

0

4

2

0

4

2

0

4

2

0

4

2

0

4

2

0

2

1

0

1

0

1

0

1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

TIME (ms)

EN1

PE_GOOD_BUCK1

EN2

PE_GOOD_BUCK2

EN3

PE_GOOD_BUCK3

PG1

PG2

PG3

NOTES

1. PE_GOOD_BUCKx ARE INTERNAL SIGNALS.

BUCK1

AGND

FB1

SW1

R1

R2

VOUT1

VOUT1

VIN1

L1

1µH

C2

22µF

+ 1

R1

R2