ADP1879ACPZ-0.3-R7 datasheet(25/40 Pages) AD

25 / 40 page

Data Sheet

ADP1878/ADP1879

Rev. A | Page 25 of 40

Ceramic capacitors are known to have low ESR. However, there

is a trade-off in using the popular X5R capacitor technology

because as much as 80% of its capacitance may be lost due to

derating as the voltage applied across the capacitor is increased

(see Figure 82). Although X7R series capacitors can also be

used, the available selection is limited to 22 μF maximum.

Figure 82. Capacitance vs. DC Voltage Characteristics for Ceramic Capacitors

Electrolytic capacitors satisfy the bulk capacitance requirements

for most high current applications. However, because the ESR of

electrolytic capacitors is much higher than that of ceramic capaci-

tors, mount several MLCCs in parallel with the electrolytic

capacitors to reduce the overall series resistance.

COMPENSATION NETWORK

Due to its current-mode architecture, the ADP1878/ADP1879

require Type II compensation. To determine the component

values needed for compensation (resistance and capacitance

values), it is necessary to examine the overall loop gain (H) of the

converter at the unity-gain frequency (fSW/10) when H = 1 V/V:

1 VV

⁄

Examining each variable at high frequency enables the unity-

gain transfer function to be simplified to provide expressions

for the RCOMP and CCOMP component values.

Output Filter Impedance (ZFILT)

Examining the transfer function of the filter at high frequencies

simplifies to

at the crossover frequency (s = 2πfCROSS). ESR is the equivalent

series resistance of the output capacitors.

Error Amplifier Output Impedance (ZCOMP)

Assuming CC2 is significantly smaller than CCOMP, CC2 can be

omitted from the output impedance equation of the error

amplifier. The transfer function simplifies to

and

where fZERO, the zero frequency, is set to be 1/4th of the crossover

frequency for the ADP1878.

Error Amplifier Gain (Gm)

The error amplifier gain (transconductance) is

Gm = 500 μA/V (μs)

Current-Sense Loop Gain (GCS)

The current-sense loop gain is

⁄

where:

ACS (V/V) is programmable for 3 V/V, 6 V/V, 12 V/V, and 24 V/V

(see the Programming Resistor (RES) Detect Circuit and Valley

Current-Limit Setting sections).

RON is the channel impedance of the low-side MOSFET.

Crossover Frequency

The crossover frequency is the frequency at which the overall

loop (system) gain is 0 dB (H = 1 V/V). It is recommended for

current-mode converters, such as the ADP1878, that the user set

the crossover frequency between 1/10th and 1/15th of the switching

frequency.

The relationship between CCOMP and fZERO (zero frequency) is as

follows:

The zero frequency is set to 1/4th of the crossover frequency.

Combining all of the above parameters results in

where ESR is the equivalent series resistance of the output

capacitors.

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

DC VOLTAGE (VDC)

X7R (50V)

X5R (25V)

X5R (16V)

10µF TDK 25V, X7R, 1210 C3225X7R1E106M

22µF MURATA 25V, X7R, 1210 GRM32ER71E226KE15L

47µF MURATA 16V, X5R, 1210 GRM32ER61C476KE15L

ADP1879ACPZ-0.3-R7 датащи(PDF) 25 Page - Analog Devices

ADP1879ACPZ-0.3-R7 датащи(HTML) 25 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 33 34 35 36 37 38 39 40

датащи скачать

ссылки URL