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LT3740

3740fc

APPLICATIONS INFORMATION

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an amount

equal to

ΔILOAD*(ESR), where ESR is the effective series

resistance of COUT. ΔILOAD also begins to charge or

discharge COUT generating a feedback error signal used

by the regulator to return VOUT to its steady-state value.

During this recovery time, VOUT can be monitored for

overshoot or ringing that would indicate a stability problem.

The VC pin external components shown in Figure 2 will

provide adequate compensation for most applications. For

a detailed explanation of switching control loop theory see

Application Note 76.

Step-Up Converter Inductor Selection

The step-up converter in the LT3740 provides a BIAS voltage

about 7V higher than the input voltage VIN for the top

MOSFET drive and most of the internal controller circuitry.

The step-up converter has a current limit of 400mA.

An inductor ripple current from 100mA to 200mA is a

reasonable design for the converter. For this consideration,

a 22μH or 47μH inductor is recommended for most of the

LT3740 applications. Small size and high efﬁciency are the

major concerns. Inductors with low core losses and small

DCR at 1MHz are good choices. Some inductors in this

category with small size are listed in Table 2.

Table 2. Recommended Inductors for Step-Up Converter

PART NUMBER

DCR (Ω)

CURRENT

RATING (MA) MANUFACTURER

LQH3C220

0.71

250

Murata

814-237-1431

www.murata.com

ELT5KT-220

0.9

420

Panasonic

714-373-7334

www.panasonic.com

CDRH3D16-220

CR32-470

0.43

0.97

400

330

Sumida

847-956-0666

www.Sumida.com

The step-up converter inductor current is the greatest

when the input voltage is the lowest. Larger Cgs of the

MOSFETs results in large inductor current. Connecting

the BGDP pin to the BIAS pin also greatly increases the

inductor current. The saturation current of the inductor

needs to cover the maximum input current.

The step-up inductor current decreases as the input voltage

increases. For high input voltages, the step-up converter

will begin skipping pulses. Although this will result in some

low frequency ripple, the BIAS voltage remains regulated

on an average basis, and the step-down controller opera-

tion is not affected.

For VIN higher than 10V step-up inductor saturation

current should be higher than 400mA. For VIN lower than

10V, a lower current rating inductor could be used. The

inductor RMS current should be higher than 250mA, and

the inductance should not be less than 10μH at 400mA.

Step-Up Converter Capacitor Selection

Thesmallsizeofceramiccapacitorsmakesthemidealforthe

output of the LT3740 step-up converter. X5R and X7R types

are recommended because they retain their capacitance

over wider voltage and temperature ranges than other types

such as Y5V or Z5U. A 1μF capacitor is recommended for

the output of the LT3740 step-up converter.

Table 3. Recommended Ceramic Capacitor Manufacturers

MANUFACTURER

PHONE

URL

Taiyo Yuden

408-573-4150

www.t-yuden.com

Murata

814-237-1431

www.murata.com

Kemet

408-986-0424

www.kemet.com

Design Example

As a design example, take a supply with the following

speciﬁcations: VIN = 7V to 20V (15V nominal), VOUT = 2.5V

± 5%, IOUT(MAX) = 10A. First, choose the inductor for about

30% ripple current at nominal VIN:

2.5V

(300kHz) • 0.3 • 10A

•1–

2.5V

15V

⎛

⎝⎜

⎞

⎠⎟

= 2.3μH

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