LT3668EMSE#PBF Datasheet LT3668IMSE#PBF, LT3668EMSE#PBF, LT3668HMSE#PBF | P19-P21

LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

Input Capacitance and Stability

Each LDO is stable with an input capacitor typically between

1μF and 10μF. This input capacitor must be placed as close

as possible to the corresponding input pin. Applications

operating with smaller input to output differential voltages

and that experience large load transients may require a

higher input capacitor value to prevent input voltage droop

and letting the regulator enter dropout.

Very low ESR ceramic capacitors may be used. However,

in cases where long wires connect the power supply to

the LDOs input and ground, use of low value input capaci

tors may

result in instability. The resonant LC tank circuit

formed

by the wire inductance and the input capacitor is

the cause and not a result of LDO instability.

The minimum input capacitance needed to stabilize the

application also varies with power supply output imped

ance variations

Placing additional capacitance on an LDO’s

output also helps. However, this requires an order of

magnitude more capacitance in comparison with additional

input bypassing. Series resistance between the supply and

an LDO’s input also helps stabilize the application; as little

as 0.1Ω to 0.5Ω suffices. This impedance dampens the

LC tank circuit at the expense of

dropout voltage. A better

alternative is to use higher ESR tantalum or electrolytic

capacitors at the input in place of ceramic capacitors.

Output Capacitance, Transient Response, Stability

Each LT3668’s LDO is stable with a wide range of output

capacitors. The ESR of the output capacitor affects stabil

ity, most notably with small capacitors. Use a minimum

output capacitor of 10μF to prevent oscillations. The ESR

of the output capacitor must not exceed 3Ω.

The LT3668 is a micropower device and output load

transient response is a function of output capacitance.

Larger values of output capacitance decrease the peak

deviations and provide improved transient response for

larger load current changes, especially for low output

voltages. Bypass capacitors, used to decouple individual

components powered by the LT3668, increase the effec

tive output capacitor value. For applications with large

load current transients, a low ESR ceramic capacitor in

parallel with a bulk tantalum capacitor often provides an

optimally damped response.

Note that some ceramic capacitors have a piezoelectric

response. A piezoelectric device generates voltage across

its terminals due to mechanical stress, similar to the way

a piezoelectric accelerometer or microphone works. For

a ceramic capacitor, the stress is induced by vibrations in

the

system or thermal transients. The resulting voltages

produced cause appreciable amounts of noise. A ceramic

capacitor produced the trace in Figure 5 in response to light

tapping from a pencil. Similar vibration induced behavior

can masquerade as increased output voltage noise.

Figure 5. Noise Resulting from Tapping On a Ceramic Capacitor

OUT2

1mV/DIV

3668 F05

2ms/DIV

OUT2

= 5V

OUT2

= 10µF

LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

External Programmable Current Limit, Enable

Each EN/ILIM pin (EN2/ILIM2 and EN3/ILIM3) is the col-

lector of

a PNP which mirrors the corresponding LDO’s

output at a ratio of 1:799 (see Block Diagram). The EN2/

ILIM2

and EN3/ILIM3 pins are also the inputs to preci

sion current limit amplifiers. If an output load increases

to the point where it causes the corresponding current

limit amplifier input voltage to reach 0.4V, the current

limit amplifier takes control of output regulation so that

its input clamps at 0.4V, regardless of the output voltage.

The current limit threshold (I

LIMIT

) of an LDO is set by

attaching a resistor (R

IMAX

) from the corresponding EN/

ILIM pin to ground:

IMAX

799•0.4V

LIM

–80Ω

In order to maintain stability, each EN/ILIM pin requires

a 47nF capacitor from that pin to ground.

In cases where the input to output voltage differential

exceeds 10V, foldback current limit will lower the inter

nal current level limit, possibly causing it to preempt the

external

programmable current limit. See the Internal

Current Limit vs Input/Output Differential graph in the

Typical Performance Characteristics section.

If an external current limit is not needed, the correspond

ing EN

/ILIM pin must be connected to ground, in which

case no capacitor is required.

Each

LDO can be individually shut down by pulling its EN/

ILIM pin above 1.2V (1V typical). Note that in this case this

pin will draw up to 500µA in certain operating conditions

until the LDO is shut down, which the circuit driving this

pin must be able to deliver. When an EN/ILIM pin is only

used to enable/disable an LDO, no capacitor is required

on this pin.

Overload Recovery

Each LDO of the LT3668 has a safe operating area pro

tection, which

decreases current limit as input-to-output

voltage increases, and keeps the power transistor inside

a safe operating region for all

values of input-to-output

voltage.

Each LDO provides some output current at all

values of input-to-output voltage up to the device break-

down. When power is first applied to an LDO, the input

voltage

rises and the output follows the input; allowing the

regulator to start-up into very heavy loads. During start-up,

as the input voltage is rising, the input-to-output voltage

differential is small, allowing the regulator to supply large

output currents. With a high input voltage, a problem can

occur wherein the removal of an output short will not allow

the output to recover. The problem occurs with a heavy

output load when the input voltage is high and the output

voltage is low. Common situations are: immediately after

the removal of a short-circuit or if an LDO is enabled via

its EN/ILIM pin after the input voltage is already turned

on. In such cases, the regulator would have to operate its

power device outside its safe operating are (high voltage

and high current) in order to bring up the output voltage.

Since this is prevented by the safe operating area protec

tion, the output gets stuck at a low voltage. Essentially,

the

load line for such a load intersects the output current

curve at two points, resulting in two stable output operating

points for the regulator. With this double intersection, the

input power supply needs to be cycled down to zero and

brought up again to make the output recover.

Protection Features

The LT3668 LDO’s protect against reverse-input voltages,

reverse-output voltages and reverse output-to-input volt

ages. Current limit protection and thermal overload protec-

tion protect the LDOs against current overload conditions

their outputs. For normal operation, do not exceed the

maximum operating junction temperature. The LT3668 IN2

pin withstands reverse voltages of 45V. The device limits

current flow to less than 300μA (typically less than 10μA)

and no negative voltages appear at OUT2. The LDOs incur

no damage if their outputs are pulled below ground. If an

input is left open circuit or grounded, the corresponding

output can be pulled below ground by 45V. No current

flows through the pass transistor from the output. If the

input is powered by a voltage source, the output sources

current equal to its current limit capability and the LT3668

protects itself by thermal limiting. Note

that the externally

programmable current limit is less accurate if the output

is pulled below ground.

LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

COMMON

Ceramic Capacitor Characteristics

Give extra consideration to the use of ceramic capacitors.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with different behavior across tempera

ture and

applied voltage. The most common dielectrics

are

specified with EIA temperature characteristic codes

of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics

provide high C-V products in a small package at low cost,

but exhibit strong voltage and temperature coefficients, as

can be seen for Y5V in Figures 6 and 7. When used with

a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an

effective value as low as 1μF to 2μF for the DC bias voltage

applied, and over the operating temperature range. The

X5R and X7R dielectrics yield much more stable charac

teristics and

are more suitable for use as input and output

capacitors.

The X7R type works over a wider temperature

range and has better temperature stability, while the X5R

is less expensive and is available in higher values. Still

exercise care when using X5R and X7R capacitors; the

X5R and X7R codes only specify operating temperature

range and maximum capacitance change over temperature.

Capacitance change due to DC bias with X5R and X7R

capacitors is better than Y5V and Z5U capacitors, but can

still be significant enough to drop capacitor values below

appropriate levels. Capacitor DC bias characteristics tend

to improve as component case size increases, but expected

capacitance at operating voltage should be verified.

Figure 6. Ceramic Capacitor DC Bias Characteristics

Figure 7. Ceramic Capacitor Temperature Characteristics

DC BIAS VOLTAGE (V)

CHANGE IN VALUE (%)

3668 F06

–20

–40

–60

–80

–100

2 6

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

25 75

3668 F07

–25 0

50 100 125

Y5V

CHANGE IN VALUE (%)

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

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

LT3668EMSE#PBF

Mfr. #:

Buy LT3668EMSE#PBF

Manufacturer:

Analog Devices / Linear Technology

Description:

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

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

LT3668EMSE#PBF

LT3668EMSE#PBF

Products related to this Datasheet