ADP1850ACPZ-R7 datasheet(12/32 Pages) AD

12 / 32 page

ADP1850

Data Sheet

Rev. C | Page 12 of 32

THEORY OF OPERATION

The ADP1850 is a current mode, dual-channel, step-down

switching controller with integrated MOSFET drivers for external

N-channel synchronous power MOSFETs. The two outputs are

phase shifted 180°. This reduces the input RMS ripple current,

thus minimizing required input capacitance. In addition, the

two outputs can be combined for dual-phase PWM operation

that can deliver more than 50 A output current and the two

channels are optimized for current sharing.

The ADP1850 can be set to operate in pulse skip high efficiency

mode (power saving mode) under light load or in forced PWM.

The integrated boost diodes in the ADP1850 reduce the overall

system cost and component count. The ADP1850 includes

programmable soft start, output overvoltage protection, program-

mable current limit, power good, and tracking function. The

ADP1850 can be set to operate in any switching frequency

between 200 kHz and 1.5 MHz with one external resistor.

CONTROL ARCHITECTURE

The ADP1850 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 22. 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 error voltage from the COMPx pin (from error

amplifier in Figure 22).

OSC

ACS

VCS

VIN

RRAMP

IRAMP

FROM

ERROR AMP

DRIVERS

FROM

LOW-SIDE

MOSFET

Figure 22. Simplified Control Architecture

As shown in Figure 22, 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

RAMPx 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 fSW values are shown in Table 4, and a graph-

ical relationship is shown in Figure 23. 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 oscil-

lator frequency to 300 kHz or 600 kHz, respectively. For other

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

and fSW can be obtained from Figure 23, or use the following

empirical formula to calculate these values:

RFREQ (kΩ) = 96,568 × fSW (kHz)−1.065

Table 4. Setting the Oscillator Frequency

FREQ

SW (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

fSW (kHz)

RFREQ (kΩ) = 96,568 fSW (kHz)–1.065

Figure 23. RFREQ vs. fSW

ADP1850ACPZ-R7 数据表(PDF) 12 Page - Analog Devices

ADP1850ACPZ-R7 数据表(HTML) 12 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载