ADP1870ARMZ-0.3-R7 datasheet(25/44 Pages) AD

25 / 44 page

ADP1870/ADP1871

Rev. 0 | Page 25 of 44

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

trade-off of using X5R technology is that up to 80% of its capaci-

tance might be lost due to derating as the voltage applied across

the capacitor is increased (see Figure 80). Although X7R series

capacitors can also be used, the available selection is limited to

only up to 22 μF.

–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

Figure 80. Capacitance vs. DC Voltage Characteristics for Ceramic Capacitors

Electrolytic capacitors satisfy the bulk capacitance requirements

for most high current applications. Because the ESR of electrolytic

capacitors is much higher than that of ceramic capacitors, when

using electrolytic capacitors, several MLCCs should be mounted

in parallel to reduce the overall series resistance.

COMPENSATION NETWORK

Due to their current-mode architecture, the ADP1870/ADP1871

require Type II compensation. To determine the component

values needed for compensation (resistance and capacitance

values), it is necessary to examine the converter’s overall loop

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

FILT

COMP

REF

OUT

= V/V

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 filter’s transfer function at high frequencies

simplifies to

OUT

FILTER

at the crossover frequency (s = 2πfCROSS).

Error Amplifier Output Impedance (ZCOMP)

Assuming that CC2 is significantly smaller than CCOMP, CC2 can

be omitted from the output impedance equation of the error

amplifier. The transfer function simplifies to

CROSS

ZERO

CROSS

COMP

)

(

and

CROSS

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

frequency for the ADP1870.

Error Amplifier Gain (GM)

The error amplifier gain (transconductance) is

= 500 μA/V

Current-Sense Loop Gain (GCS)

The current-sense loop gain is

(A/V)

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 lower-side MOSFET.

Crossover Frequency

The crossover frequency is the frequency at which the overall

loop (system) gain is 0 dB (H = 1 V/V). For current-mode

converters, such as the ADP1870, it is recommended that the

user set the crossover frequency between 1/10th and 1/15th of the

switching frequency.

CROSS

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

follows:

COMP

ZERO

)

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

Combining all of the above parameters results in

REF

OUT

CROSS

ZERO

CROSS

COMP

ZERO

COMP

ADP1870ARMZ-0.3-R7 датащи(PDF) 25 Page - Analog Devices

ADP1870ARMZ-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 41 42 43 44

датащи скачать

ссылки URL