ADP1877ACPZ-R7 datasheet(13/32 Pages) AD

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ADP1877

Rev. 0 | Page 13 of 32

THEORY OF OPERATION

The ADP1877 is a current mode (using ADI proprietary

FlexMode® architecture), dual-channel, step-down switching

controller with integrated MOSFET drivers that drive N-channel

synchronous power MOSFETs. The two outputs are phase shifted

180°. This reduces the input RMS current, thus minimizing

required input capacitance.

The ADP1877 can be set to operate in pulse skip high efficiency

mode under light load or in forced PWM. The integrated boost

diodes in the ADP1877 reduce the overall system cost and

component count. The ADP1877 includes programmable soft

start, output overvoltage protection, programmable current

limit, power good, and tracking function. The ADP1877 can be

set to operate in any switching frequency between 200 kHz and

1.5 MHz with one external resistor.

CONTROL ARCHITECTURE

The ADP1877 is based on a fixed frequency current mode

PWM control architecture. The inductor current is sensed by

the voltage drop measured across the external low-side MOSFET

RDSON during the off period of the switching cycle (valley inductor

current). The current sense signal is further processed by the

current sense amplifier. The output of the current sense amplifier is

held, and the emulated current ramp is multiplexed and fed into

the PWM comparator as shown in Figure 23. The valley current

information is captured at the end of the off period, and the

emulated current ramp is applied at that point when the next on

cycle begins. An error amplifier integrates the error between the

feedback voltage and the generated the error voltage from the

COMP pin (from error amp in Figure 23).

OSC

ACS

VCS

VIN

RRAMP

IRAMP

FROM

ERROR AMP

DRIVERS

FROM

LOW SIDE

MOSFET

Figure 23. Simplified Control Architecture

As shown in Figure 23, the emulated current ramp is generated

inside the IC but offers programmability through the RAMPx

pin. Selecting an appropriate value resistor from VIN to the

RAMP pin programs a desired slope compensation value and, at

the same time, provides a feed forward feature. The benefits

realized by deploying this type of control scheme are that there

is no need to worry about the turn-on current spike corrupting

the current ramp. Also, the current signal is stable because the

current signal is sampled at the end of the turn-off period,

which gives time for the switch node ringing to settle. Other

benefits of using current mode control scheme still apply, such

as simplicity of loop compensation. Control logic enforces

antishoot-through operation to limit cross conduction of the

internal drivers and external MOSFETs.

OSCILLATOR FREQUENCY

The internal oscillator frequency, which ranges from 200 kHz to

1.5 MHz, is set by an external resistor, RFREQ, at the FREQ pin.

Some popular fOSC values are shown in Table 4, and a graphical

relationship is shown in Figure 24. For instance, a 78.7 kΩ

resistor sets the oscillator frequency to 800 kHz. Furthermore,

connecting FREQ to AGND or FREQ to VCCO sets the

oscillator frequency to 300 kHz or 600 kHz, respectively. For

other frequencies that are not listed in Table 4, the values of

RFREQ and fOSC can be obtained from Figure 24, or use the

following empirical formula to calculate these values:

065

)

(

96568

)

(

−

kHz

OSC

FREQ

Table 4. Setting the Oscillator Frequency

RFREQ

fOSC (Typical)

332 kΩ

200 kHz

78.7 kΩ

800 kHz

60.4 kΩ

1000 kHz

51 kΩ

1200 kHz

40.2 kΩ

1500 kHz

FREQ to AGND

300 kHz

FREQ to VCCO

600 kHz

110

160

210

260

310

360

410

100

400

700

1000

1300

1600

1900

fOSC (kHz)

RFREQ (kΩ) = 96568 fOSC (kHz)–1.065

Figure 24. RFREQ vs. fOSC

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