ADP1874ARQZ-0.3-R7 datasheet(28/44 Pages) AD

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ADP1874/ADP1875

Rev. 0 | Page 28 of 44

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

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

because up to 80% of its capacitance may be lost due to derating

as the voltage applied across the capacitor is increased (see

Figure 86). Although X7R series capacitors can also be used, the

available selection is limited to 22 μF maximum.

–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 86. 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

capacitors, several MLCCs should be mounted in parallel with

the electrolytic capacitors to reduce the overall series resistance.

COMPENSATION NETWORK

Due to its current-mode architecture, the ADP1874/ADP1875

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

OUT

REF

= 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

ESR

)

(

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

series resistance of the output capacitors.

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

ZERO

CROSS

COMP

and

CROSS

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

frequency for the ADP1874.

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

(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). It is recommended for

current-mode converters, such as the ADP1874, that the user

set the crossover frequency between 1/10 and 1/15 the

switching frequency.

CROSS

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

follows:

COMP

ZERO

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

Combining all of the above parameters results in

()

REF

OUT

ZERO

CROSS

COMP

ESR

)

(

where ESR is the equivalent series resistance of the output

capacitors.

ZERO

COMP

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