LT3668EMSE#PBF Datasheet LT3668IMSE#PBF, LT3668EMSE#PBF, LT3668HMSE#PBF | P16-P18

LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

Figure 1. Burst Mode Operation

where f

is in MHz, and C

OUT1

is the recommended output

capacitance in μF. Use X5R or X7R types. This choice will

provide low output ripple and good transient response.

Transient performance can be improved with a higher value

capacitor if combined with a phase lead capacitor (typically

22pF) between the output and pin FB1. Note that a larger

phase lead capacitor should be used with a large output

capacitor. A lower value of output capacitor can be used to

save space and cost but transient performance will suffer.

When choosing a capacitor, look carefully through the

data sheet to find out what the actual capacitance is under

operating conditions (applied voltage and temperature).

A physically larger capacitor, or one with a higher voltage

rating, may be required. Table 3 lists several capacitor

vendors.

Table 3: Capacitor Vendors

VENDOR URL

Panasonic www.panasonic.com

Kemet www.kemet.com

Sanyo www.sanyovideo.com

Murata www.murata.com

AVX www.avxcorp.com

Taiyo Yuden www.taiyo-yuden.com

Audible Noise

Ceramic capacitors are small, robust and have very

low ESR. However, ceramic capacitors can sometimes

cause problems when used with the LT3668 due to their

piezoelectric nature. When in Burst Mode operation, the

LT3668’s switching frequency depends on the load current,

and at very light loads the LT3668 can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LT3668 operates at a lower current limit during

Burst Mode operation, the noise is typically very quiet. If

this is unacceptable, use a high performance tantalum or

electrolytic capacitor at the output.

Low Ripple Burst Mode Operation

To enhance efficiency at light loads, the LT3668 oper

ates in low ripple Burst Mode operation which keeps

the

output capacitor charged to the proper voltage while

minimizing the input quiescent current. During Burst Mode

operation, the LT3668 delivers single cycle bursts of

current to the output capacitor followed by sleep periods

where the output power is delivered to the load by the

output capacitor. Because the LT3668 delivers power

to the output

with single, low current pulses, the output

ripple

is kept below 5mV for a typical application. As the

load current decreases towards a no load condition, the

percentage of time that the LT3668 operates in sleep mode

increases and the average input current is greatly reduced

resulting in high efficiency even at very low loads. Note

that during Burst Mode operation, the switching frequency

will be lower than the programmed switching frequency.

At higher output loads (above ~50mA for the front page

application) the LT3668 will be running at the frequency

programmed by the R

resistor, and will be operating in

standard PWM mode. The transition between PWM and

low ripple Burst Mode operation is seamless, and will not

disturb the output voltage.

3668 F01

1µs/DIV

FRONT PAGE APPLICATION, V

OUT1

= 5V

5V/DIV

OUT1

5mV/DIV

100mA/DIV

LOAD

= 10mA

LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

BOOST and IN3/BD Pin Considerations

Capacitor C2 and the internal boost Schottky diode (see the

Block Diagram) are used to generate a boost voltage that

is higher than the input voltage. In most cases a 0.22μF

capacitor will work well. Figure 2 shows two ways to ar

range the

boost circuit. The BOOST pin must be more than

1.9V

above the SW pin for best efficiency. For outputs of

2.2V and above, the standard circuit (Figure 2a) is best.

For outputs between 2.2V and 2.5V, use a 0.47μF boost

capacitor. For output voltages below 2.2V, the boost diode

can be tied to the input (Figure 2b), or to another external

supply greater than 2.2V. However, the circuit in Figure

2a is more efficient because the BOOST pin current and

IN3/BD pin quiescent current come from a lower voltage

source. Also, be sure that the maximum voltage ratings

of the BOOST and IN3/BD pins are not exceeded.

The minimum operating voltage of an LT3668 applica

tion is limited by the minimum input voltage (4.3V) and

by the maximum duty cycle as outlined in a previous

section. For proper start-up, the minimum input voltage

also limited by the boost circuit. If the input voltage

is ramped slowly, the boost capacitor may not be fully

charged. Because the boost capacitor is charged with the

energy stored in the inductor, the circuit relies on some

minimum load current to get the boost circuit running

properly. This minimum load depends on input and output

voltages, and on the arrangement of the boost circuit. The

minimum load generally goes to zero once the circuit has

started. Figure3 shows a plot of minimum load to start

and to run as a function of input voltage. In many cases

the discharged output capacitor will present a load to the

switcher, which will allow it to start. The plots show the

worst-case situation where V

IN1

is ramping very slowly.

For lower start-up voltage, the boost diode can be tied to

IN1

; however, this restricts the input range to one-half of

the absolute maximum rating of the BOOST pin.

Figure 2. Two Circuits for Generating the Boost Voltage

IN3/BD

LT3668

(2a) For V

OUT1

≥ 2.2V

BOOSTIN1V

IN1

OUT1

GND

IN3/BD

LT3668

(2b) For V

OUT1

< 2.2V; V

IN1

< 25V

BOOSTIN1V

IN1

3668 F02

OUT1

GND

Figure 3. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

LOAD CURRENT I

OUT1

(mA)

INPUT VOLTAGE V

IN1

(V)

4.0

4.5

5.0

150 250 400

3668 F03a

3.5

3.0

3.5

50 100

200

300 350

TO START

TO RUN

FRONT PAGE APPLICATION

= V

IN1

, V

OUT1

= 3.3V

LOAD CURRENT I

OUT1

(mA)

INPUT VOLTAGE V

IN1

(V)

6.5

7.0

7.5

150 250 400

3668 F03b

6.0

5.5

5.0

50 100 200 300 350

TO START

TO RUN

FRONT PAGE APPLICATION

= V

IN1

, V

OUT1

= 6V

LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, the

switching regulator will tolerate a shorted

output.

There is another situation to consider in systems

where the output will be held high when the input to the

LT3668 is absent. This may occur in battery charging

applications or

in battery backup systems where a battery

or some other supply is diode ORed with the switching

regulator’s output. If the IN1 pin is allowed to float and

the EN pin is held high (either by a logic signal or because

it is tied to IN1), then the LT3668’s internal circuitry will

pull its quiescent current through the SW pin. This is fine

if the system can tolerate a few μA in this state. If the EN

pin is grounded, the SW pin current will drop to 0.7μA.

However, if the IN1 pin is grounded while the output is

held high, regardless of EN, parasitic diodes inside the

LT3668 can pull current from the output through the SW

pin and the IN1 pin. Figure 4 shows a circuit that will run

only when the input voltage is present

and that protects

against a shorted or reversed input.

LDOs

Adjustment Inputs

Each LDO output voltage of the LT3668 follows the voltage

at the corresponding adjustment pin ADJ2/ADJ3. Each

adjustment pin is pulled down by an internal current source

(typically 200nA at 25°C). This current must be taken into

consideration if an adjustment pin is to be driven by a high

impedance resistive divider.

Even if the voltage at ADJ2/ADJ3 is below the minimum input

voltage, the corresponding output will always be regulated

to a voltage equal or below the voltage at ADJ2/ADJ3.

Any noise present at an adjustment pin is transferred to the

corresponding output, in particular low frequency noise.

See the LDO transfer function in the Typical Performance

Characteristics section. Reference voltage noise can be

reduced by connecting a capacitor from ADJ2/ADJ3 to

ground. However, if the reference voltage is derived from

the resistive divider of the switching regulator as shown

in the application on page 1, no such bypass capacitor

is allowed as it would impair the switching regulator's

stability.

Figure 4. Diode D1 Prevents a Shorted Input from Discharging

a Backup Battery Tied to the Output. It Also Protects the Circuit

from a Reversed Input, in Which Case the Resistor at the EN Pin

Limits the Current Drawn from That Pin. The LT3668 Runs Only

When the Input Is Present

IN3/BD

LT3668

BOOSTIN1

OUT

BACKUP

3668 F04

MBRS140

FB1

GND

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P28

LT3668EMSE#PBF

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Manufacturer:

Analog Devices / Linear Technology

Description:

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

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LT3668EMSE#PBF

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