ADP1879ACPZ-0.6-R7 datasheet(22/40 Pages) AD

22 / 40 page

ADP1878/ADP1879

Data Sheet

Rev. A | Page 22 of 40

Figure 76. 10 mV Offset to Ensure Prevention of Negative Inductor Current

The system remains in idle mode until the output voltage drops

below regulation. Next, a PWM pulse is produced, turning on the

high-side MOSFET to maintain system regulation. The ADP1879

does not have an internal clock; it switches purely as a hysteretic

controller, as described in this section.

TIMER OPERATION

The ADP1878/ADP1879 employ a constant on-time architecture,

which provides a variety of benefits, including improved load

and line transient response when compared with a constant

(fixed) frequency current-mode control loop of comparable

loop design. The constant on-time timer, or tON timer, senses

the high-side input voltage (VIN) and the output voltage (VOUT)

using SW waveform information to produce an adjustable one

shot PWM pulse. The pulse varies the on-time of the high-side

MOSFET in response to dynamic changes in input voltage, output

voltage, and load current conditions to maintain output regula-

tion. The timer generates an on-time (tON) pulse that is inversely

proportional to VIN.

where K is a constant that is trimmed using an RC timer product

for the 300 kHz, 600 kHz, and 1.0 MHz frequency options.

Figure 77. Constant On-Time Time

The constant on-time (tON) is not strictly constant because it

varies with VIN and VOUT. However, this variation occurs in such

a way as to keep the switching frequency virtually independent

of VIN and VOUT.

The tON timer uses a feedforward technique that, when applied

to the constant on-time control loop, makes it a pseudo fixed

frequency to a first-order approximation.

Second-order effects, such as dc losses in the external power

MOSFETs (see the Efficiency Consideration section), cause some

variation in frequency vs. load current and line voltage. These

effects are shown in Figure 23 to Figure 34. The variations in

frequency are much reduced compared with the variations

generated if the feedforward technique is not used.

The feedforward technique establishes the following relationship:

where fSW is the controller switching frequency (300 kHz,

600 kHz, and 1.0 MHz).

The tON timer senses VIN and VOUT to minimize frequency

variation as previously explained. This provides pseudo fixed

frequency as explained in the Pseudo Fixed Frequency section.

To allow headroom for VIN and VOUT sensing, adhere to the

following equations:

VREG ≥ VIN/8 + 1.5

VREG ≥ VOUT/4

For typical applications where VREG is 5 V, these equations are

not relevant; however, for lower VREG inputs, care may be required.

PSEUDO FIXED FREQUENCY

The ADP1878/ADP1879 employ a constant on-time control

scheme. During steady state operation, the switching frequency

stays relatively constant, or pseudo fixed. This is due to the one

shot tON timer that produces a high-side PWM pulse with a

fixed duration, given that external conditions such as input

voltage, output voltage, and load current are also at steady state.

During load transients, the frequency momentarily changes for

the duration of the transient event so that the output comes

back within regulation quicker than if the frequency were fixed,

or if it were to remain unchanged. After the transient event is

complete, the frequency returns to a pseudo fixed value.

To illustrate this feature more clearly, this section describes one

such load transient event—a positive load step—in detail. During

load transient events, the high-side driver output pulse width

stays relatively consistent from cycle to cycle; however, the off

time (DRVL on time) dynamically adjusts according to the

instantaneous changes in the external conditions mentioned.

When a positive load step occurs, the error amplifier (out of phase

with the output, VOUT) produces new voltage information at its

output (COMP). In addition, the current sense amplifier senses

new inductor current information during this positive load

transient event. The output voltage reaction of the error amplifier is

compared with the new inductor current information that sets

the start of the next switching cycle. Because current information

is produced from valley current sensing, it is sensed at the down

ramp of the inductor current, whereas the voltage loop information

HS AND LS

IN IDLE MODE

10mV = RON × ILOAD

ZERO-CROSS COMPARATOR

DETECTS 10mV OFFSET AND

TURNS OFF LS

ILOAD

tON

ANOTHER

tON EDGE IS

TRIGGERED WHEN VOUT

FALLS BELOW REGULATION

R (TRIMMED)

VREG

tON

VIN

INFORMATION

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