LT3150CGN#TRPBF【PDF 12/20 页】IC REG CTRLR SGL POS ADJ 16-SSOP

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LT3150

3150f

APPLICATIO S I FOR ATIO

holds internal comparator COMP3s output low. This

comparator circuit, now enabled, monitors the GATE pin

and detects saturation at the positive rail. When a saturated

condition is detected, COMP3 activates the shutdown timer.

Once the time-out period occurs, the output is shut down

and latched off. The operation of resetting the latch remains

the same. Note that this technique does not limit the FET

current during the time-out period. The output current is

only limited by the input power supply and the input/out-

put impedance. Setting the timer to a short period in this

mode of operation keeps the external MOSFET within its

SOA (safe operating area) boundary and keeps the

MOSFETs temperature rise under control.

Unique circuit design incorporated into the LT3150 allevi-

ates all concerns about power supply sequencing. The

issue of power supply sequencing is an important topic as

the typical LT3150 application has two separate power

supply inputs, V

IN1

and V

IN2

. If the V

IN2

supply voltage is

slow in ramping up or is held off by SHDN1, insufficient

MOSFET gate drive exists and therefore, the output

voltage does not come up. This statement is true as long

as the V

IN1

input voltage is lower than the threshold of the

external MOSFET. Prior to the boost converter powering

up, V

IN2

equals V

IN1

due to the DC path present

through the boost inductor. If V

IN1

is high enough, the

MOSFET turns on and pulls the output voltage up. If this

situation exists and the output must be held off, then

pulling the SHDN2 pin high actively holds the output off.

Pull the SHDN2 pin low to allow start-up, as the SHDN2

high logic state is a latched condition.

If V

IN2

is present, but the V

IN1

supply voltage tied to the

POS

pin is slow in ramping, then the feedback loop wants

to drive the GATE pin to the positive V

IN2

rail. This results

in a large current as the V

IN1

supply ramps up. However,

undervoltage lockout circuit COMP2, which monitors the

POS

supply voltage, holds Q6 on and pulls the COMP pin

low until the I

POS

voltage increases to greater than the

internal 1.21 reference voltage. The undervoltage lockout

circuit then smoothly releases the COMP pin and allows

the output voltage to come up in dropout from the input

supply voltage. An additional benefit derived from the

speed of the LT3150 feedback loop is that turn-on

overshoot is virtually nonexistent in a properly compen-

sated system.

BOOST REGULATOR COMPONENT SELECTION

Diode

Linear Technology recommends the use of a Schottky

diode with the LT3150. For input supply voltages less than

2V, the Motorola MBR0520 or equivalent is a good choice

due to its small size, low cost and low forward voltage. The

average diode current equals the V

IN2

supply current of

12mA typically. The peak diode current equals the peak

switch current, which in these low input-to-output voltage

applications ranges from 100mA to 200mA.

The diodes forward voltage during its conduction period

directly affects the duty cycle of the boost converter. These

low input-to-output voltage applications require the boost

converter to operate at duty cycles close to the maximum

and the difference of a few hundred millivolts in the diode

forward voltage results in a duty cycle difference of several

percent. For supply voltages greater than 2V, a 1N4148 is

suitable and lowers cost.

Inductor

Use inductors with a saturation current rating (where

inductance is approximately 70% of zero current induc-

tance) of 0.2A or greater. Also, choose an inductor with a

DCR of 2.5& or less. The inductors DCR also affects the

boost converters duty cycle. A larger DCR value increases

the required duty cycle. An inductance value between

4.7礖 and 10礖 works well in most applications.

Table 1 lists several 10礖 inductors that work with the

LT3150, although this is by no means an exhaustive list.

Many magnetic vendors have components suitable for use

in this boost application.

Input Capacitor

The input bypass capacitors serve as the reservoir capaci-

tor for the boost regulator, the linear regulator and what-

ever other system circuitry the input supply powers.

Therefore, the input capacitor network is most likely

distributed along the input supply PCB plane.

However, the switching of current at high speed by the

boost regulator mandates a local bypass capacitor at the

IN1

pin. Place this input capacitor physically close to the

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