LT3667HMSE#PBF Datasheet LT3667IMSE#PBF, LT3667IUDD#PBF, LT3667HUDD#PBF | P16-P18

LT3667

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APPLICATIONS INFORMATION

SWITCHING REGULATOR

FB1 Resistor Network

The switching regulator output voltage of the LT3667 is

programmed with a resistor divider between the output

of the switching regulator and the FB1 pin. Choose the

resistor values according to:

R1=R2

OUT1

1.2V

–1













Reference designators refer to the Block Diagram of the

LT3667. 1% resistors are recommended to maintain output

voltage accuracy. Note that choosing larger resistors will

decrease the quiescent current of the application circuit.

Setting the Switching Frequency

The LT3667 regulator uses a constant frequency PWM

architecture that can be programmed to switch from

250kHz to 2.2MHz by using a resistor tied from the RT

pin to ground. Table 1 shows the necessary R

value for

a desired switching frequency.

Table 1: Switching Frequency vs R

Value

SWITCHING FREQUENCY (MHz) R

VALUE (kΩ)

0.25 475

0.3 383

0.4 274

0.5 215

0.6 174

0.8 124

1 95.3

1.2 75

1.4 61.9

1.6 51.1

1.8 43.2

2 37.4

2.2 32.4

Operating Frequency Trade-Offs

Selection of the operating frequency is a trade-off between

efficiency, component size, minimum dropout voltage, and

maximum input voltage. The advantage of high frequency

operation is that smaller inductor and capacitor values may

be used. The disadvantages are lower efficiency, lower

maximum input voltage, and higher dropout voltage. The

highest acceptable switching frequency (f

SW(MAX)

) for a

given application can be calculated as follows:

SW(MAX)

OUT1

+ V

ON(MIN)

IN1

– V

+ V

( )

where V

IN1

is the typical input voltage, V

OUT1

is the output

voltage, V

is the catch diode drop (~0.5V) and V

is the

internal switch drop (~0.5V at max load). This equation

shows that slower switching frequency is necessary to

accommodate high V

IN1

OUT1

ratio.

Lower frequency also allows a lower dropout voltage. Input

voltage range depends on the switching frequency because

the LT3667 switch has finite minimum on and off times.

The switch can turn on for a minimum of ~150ns and turn

off for a minimum of ~170ns (note that the minimum on-

time is a strong function of temperature). The minimum

and maximum duty cycles that can be achieved taking

minimum on and off times into account are:

MIN

= f

• t

ON(MIN)

MAX

= 1 − f

• t

OFF(MIN)

where f

is the switching frequency, t

ON(MIN)

is the

minimum switch on-time (~150ns), and t

OFF(MIN)

is the

minimum switch off-time (~170ns). These equations show

that the duty cycle range increases when the switching

frequency is decreased.

A good choice of switching frequency should allow an

adequate input voltage range (see Input Voltage Range

section) and

keep the inductor and capacitor values small.

LT3667

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APPLICATIONS INFORMATION

Input Voltage Range

The minimum input voltage is determined by either the

LT3667’s minimum operating voltage of 4.3V or by its

maximum duty cycle (as discussed in the previous sec

tion). The minimum input voltage due to duty cycle is:

IN1(MIN)

OUT1

+ V

1– f

• t

OFF(MIN)

– V

+ V

where V

IN(MIN)

is the minimum input voltage, V

OUT1

is the

output voltage, V

is the catch diode drop (~0.5V), V

is the internal switch drop (~0.5V at maximum load), f

is the switching frequency, and t

OFF(MIN)

is the minimum

switch off-time (~170ns). Note that a higher switching

frequency will increase the minimum input voltage. If a

lower dropout voltage is desired, a lower switching fre

quency should be used.

The highest

allowed V

IN1

during normal operation

IN1(OP-MAX)

) is limited by minimum duty cycle and is

given by:

IN1(OP-MAX)

OUT1

+ V

• t

ON(MIN)

– V

+ V

where V

OUT1

is the output voltage, V

is the catch diode

drop (~0.5V), V

is the internal switch drop (~0.5V at

maximum load), f

is the switching frequency, and

ON(MIN)

is the minimum switch on-time (~150ns).

However, the LT3667 will tolerate inputs up to the absolute

maximum ratings of the V

IN1

and BOOST pins, regardless

of the chosen switching frequency. During such transients

where V

IN1

is higher than V

IN1(OP-MAX)

, the part will skip

pulses to maintain output regulation. The output voltage

ripple and inductor current ripple will be higher than in

normal operation. Input voltage transients of up to 60V are

also safely withstood, though the LT3667 stops switching

while V

IN1

> V

OVLO

(overvoltage lockout, 42V typical), al-

lowing the output to fall out of regulation.

During start

-up, short-circuit, or other overload conditions

the inductor peak current might reach and even exceed the

maximum current limit of the LT3667, especially in those

cases where the switch already operates at minimum on-

time. The catch diode current limit circuitry prevents the

switch from turning on again if the inductor valley current

is above 500

mA nominal.

Inductor Selection and Maximum Output Current

For

a given input and output voltage, the inductor value

and switching frequency will determine the ripple current,

which increases with higher V

IN1

or V

OUT1

and decreases

with higher inductance and higher switching frequency.

A good first choice for the inductor value is:

L = V

OUT1

+ V

( )

•

2.4

where f

is the switching frequency in MHz, V

OUT1

is the

output voltage, V

is the catch diode drop (~0.5V) and L is

the inductor value in μH. The inductor’s RMS current rating

must be greater than the maximum load current and its

saturation current should be about 30% higher. For robust

operation in fault conditions (start-up or short-circuit) and

high input voltage (>30V), the saturation current should

be above 900mA. To keep the efficiency high, the series

resistance (DCR) should be less than 0.3Ω, and the core

material should be intended for high frequency applica

tions. Table 2 lists several vendors.

Table 2. Inductor Vendors

VENDOR URL

Coilcraft www.coilcraft.com

Sumida www.sumida.com

Toko www.tokoam.com

Würth Elektronik www.we-online.com

Coiltronics www.cooperet.com

Murata www.murata.com

LT3667

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For more information www.linear.com/LT3667

APPLICATIONS INFORMATION

This simple design guide will not always result in the

optimum inductor selection for a given application. As a

general rule, lower output voltages and higher switching

frequency will require smaller inductor values. If the ap

plication requires

less than 400mA load current, then a

lesser inductor value may be acceptable. This allows the

use of a physically smaller inductor, or one with a lower

DCR resulting in higher efficiency. However, the inductance

should in general not be smaller than 10µH.

Be aware that if the inductance differs from the simple

rule above, then the maximum load current will depend

on input voltage. In addition, low inductance may result

in discontinuous mode operation, which further reduces

maximum load current. For details of maximum output

current and discontinuous mode operation, see Linear

Technology’s Application Note 44. Finally, for duty cycles

greater than 50% (V

OUT1

IN1

> 0.5), a minimum inductance

is required to avoid sub-harmonic oscillations:

MIN

= V

OUT1

+ V

( )

•

where f

is the switching frequency in MHz, V

OUT1

the output voltage, V

is the catch diode drop (~0.5V)

and L

MIN

is the inductor value in µH.

Catch Diode

The catch diode (D1 from block diagram) conducts current

only during switch off-time. Use a 1A Schottky diode for

best performance.

Peak reverse voltage is equal to V

IN1

if it is below the

overvoltage protection threshold. This feature keeps the

switch off for V

IN1

> OVLO (44V maximum). For inputs up

to the maximum operating voltage of 40V, use a diode with

a reverse voltage rating greater than the input voltage. If

transients at the input of up to 60V are expected, use a diode

with a reverse voltage rating only higher than the maximum

OVLO of 44V. If operating at high ambient temperatures,

consider using a Schottky with low reverse leakage. For

example, Diodes Inc. SBR1U40LP or DFLS160, ON Semi

MBRM140, and Central Semiconductor CMMSH1-60 are

good choices for the catch diode.

Input Capacitor

Bypass the input of the LT3667 circuit with a ceramic

capacitor of X7R or X5R type. Y5V types have poor

performance over temperature and applied voltage,

and

should

not be used. A 1μF to 4.7μF ceramic capacitor is

adequate to bypass the LT3667 and will easily handle

the ripple current. Note that a larger input capacitance

is required when a lower switching frequency is used

(due to longer on-times). If the input power source has

high impedance, or there is significant inductance due to

long wires or cables, additional bulk capacitance may be

necessary. This can be provided with a low performance

electrolytic capacitor. Step-down regulators draw current

from the input supply in pulses with very fast rise and

fall times. The input capacitor is required to reduce the

resulting voltage ripple at the LT3667 and to force this

very high frequency switching current into a tight local

loop, minimizing EMI. A 1μF capacitor is capable of this

task, but only if it is placed close to the LT3667 (see the

PCB Layout section). A second precaution regarding the

ceramic input capacitor concerns the maximum input

voltage rating of the LT3667. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (under damped) tank circuit. If the LT3667 circuit

is plugged into a live supply, the input

voltage can ring to

twice its nominal value, possibly exceeding the LT3667’s

voltage rating. This situation is easily avoided (see the Hot

Plugging Safely section).

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated

by the LT3667 to produce the DC output. In this role it

determines the output ripple, and low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the switching regulator’s control loop. Ceramic

capacitors have very low equivalent series resistance

(ESR) and provide the best ripple performance. A good

starting value is:

OUT1

• f

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LT3667HMSE#PBF

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Description:

Switching Voltage Regulators 40V 400mA Step-Down Switching Regulator with Dual Fault Protected LDOs

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