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AD5737

Data Sheet

Rev. B | Page 36 of 44

DC-to-DC Converter On-Board Switch

The AD5737 contains a 0.425 Ω internal switch. The switch

current is monitored on a pulse-by-pulse basis and is limited

to 0.8 A peak current.

DC-to-DC Converter Switching Frequency and Phase

The AD5737 dc-to-dc converter switching frequency can be

selected from the dc-to-dc control register (see Table 27). The

phasing of the channels can also be adjusted so that the dc-to-dc

converters can clock on different edges. For typical applications,

a 410 kHz frequency is recommended. At light loads (low output

current and small load resistor), the dc-to-dc converter enters a

pulse-skipping mode to minimize switching power dissipation.

DC-to-DC Converter Inductor Selection

For typical 4 mA to 20 mA applications, a 10 µH inductor (such

as the XAL4040-103 from Coilcraft), combined with a switching

frequency of 410 kHz, allows up to 24 mA to be driven into a

load resistance of up to 1 kΩ with an AVCC supply of 4.5 V to

5.5 V. It is important to ensure that the inductor can handle the

peak current without saturating, especially at the maximum

ambient temperature. If the inductor enters saturation mode,

efficiency decreases. The inductance value also drops during

saturation and may result in the dc-to-dc converter circuit not

being able to supply the required output power.

DC-to-DC Converter External Schottky Diode Selection

The AD5737 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 AD5737

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 27). 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 AD5737, 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 30 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 32

and Figure 33).

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