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Data Sheet

AD5735

Rev. B | Page 43 of 48

DC-to-DC Converter External Schottky Diode Selection

The AD5735 requires an external Schottky diode for correct

operation. Ensure that the Schottky diode is rated to handle the

maximum reverse breakdown voltage expected in operation

and that the maximum junction temperature of the diode is not

exceeded. The average current of the diode is approximately

equal to the ILOAD current. Diodes with larger forward voltage

drops result in a decrease in efficiency.

DC-to-DC Converter Compensation Capacitors

Because the dc-to-dc converter operates in discontinuous conduc-

tion mode, the uncompensated transfer function is essentially a

single-pole transfer function. The pole frequency of the transfer

function is determined by the output capacitance, input and output

voltage, and output load of the dc-to-dc converter. The AD5735

uses an external capacitor in conjunction with an internal 150 kΩ

resistor to compensate the regulator loop.

Alternatively, an external compensation resistor can be used in

series with the compensation capacitor by setting the DC-DC

comp bit in the dc-to-dc control register (see Table 28). In this

case, a resistor of ~50 kΩ is recommended. The advantages of this

configuration are described in the AICC Supply Requirements—

Slewing section. For typical applications, a 10 nF dc-to-dc com-

pensation capacitor is recommended.

DC-to-DC Converter Input and Output Capacitor

Selection

The output capacitor affects the ripple voltage of the dc-to-dc

converter and indirectly limits the maximum slew rate at which

the channel output current can rise. The ripple voltage is caused

by a combination of the capacitance and the equivalent series

resistance (ESR) of the capacitor. For typical applications, a

ceramic capacitor of 4.7 µF is recommended. Larger capacitors

or parallel capacitors improve the ripple at the expense of

reduced slew rate. Larger capacitors also affect the current

requirements of the AVCC supply while slewing (see the AICC

Supply Requirements—Slewing section). The capacitance at

the output of the dc-to-dc converter should be >3 µF under all

operating conditions.

The input capacitor provides much of the dynamic current

required for the dc-to-dc converter and should be a low ESR

component. For the AD5735, a low ESR tantalum or ceramic

capacitor of 10 µF is recommended for typical applications.

Ceramic capacitors must be chosen carefully because they can

exhibit a large sensitivity to dc bias voltages and temperature.

X5R or X7R dielectrics are preferred because these capacitors

remain stable over wider operating voltage and temperature

ranges. Care must be taken if selecting a tantalum capacitor to

ensure a low ESR value.

AICC SUPPLY REQUIREMENTS—STATIC

The dc-to-dc converter is designed to supply a VBOOST_x voltage of

VBOOST_x = IOUT × RLOAD + Headroom

(2)

See Figure 51 for a plot of headroom supplied vs. output

current. Therefore, for a fixed load and output voltage, the

output current of the dc-to-dc converter can be calculated

by the following formula:

BOOST

OUT

Efficiency

Out

Power

BOOST

(3)

where:

IOUT is the output current from IOUT_x in amperes.

ηV

BOOST is the efficiency at VBOOST_x as a fraction (see Figure 53

and Figure 54).

AICC SUPPLY REQUIREMENTS—SLEWING

The AICC current requirement while slewing is greater than in

static operation because the output power increases to charge

the output capacitance of the dc-to-dc converter. This transient

current can be quite large (see Figure 79), although the methods

described in the Reducing AICC Current Requirements section

can reduce the requirements on the AVCC supply.

If not enough AICC current can be provided, the AVCC voltage

drops. Due to this AVCC drop, the AICC current required for

slewing increases further, causing the voltage at AVCC to drop

further (see Equation 3). In this case, the VBOOST_x voltage and,

therefore, the output voltage, may never reach their intended

values. Because the AVCC voltage is common to all channels, this

voltage drop may also affect other channels.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.5

1.0

1.5

2.0

2.5

TIME (ms)

AICC

IOUT

VBOOST

0mA TO 24mA RANGE

1kΩ LOAD

fSW = 410kHz

INDUCTOR = 10µH (XAL4040-103)

TA = 25°C

Figure 79. AICC Current vs. Time for 24 mA Step Through 1 kΩ Load

with Internal Compensation Resistor

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