ADP1874ARQZ-0.3-R7 datasheet(23/44 Pages) AD

23 / 44 page

ADP1874/ADP1875

Rev. 0 | Page 23 of 44

As soon as the forward current through the lower-side MOSFET

decreases to a level where

10 mV = IQ2 × RON(Q2)

the zero-cross comparator (or IREV comparator) emits a signal to

turn off the lower-side MOSFET. From this point, the slope of the

inductor current ramping down becomes steeper (see Figure 76)

as the body diode of the lower-side MOSFET begins to conduct

current and continues conducting current until the remaining

energy stored in the inductor has been depleted.

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

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. A PWM pulse is then produced, turning on the

upper-side MOSFET to maintain system regulation. The ADP1875

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

controller as described in this section.

TIMER OPERATION

The ADP1874/ADP1875 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 upper-side

MOSFET in response to dynamic changes in input voltage,

output voltage, and load current conditions to maintain output

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

inversely proportional to VIN.

OUT

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.

R (TRIMMED)

VREG

tON

VIN

INFORMATION

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, which 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, care may be required for lower VREG/VIN

inputs.

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