LT8410 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT8410

Ultralow Power Boost Converter with Output Disconnect

Linear Technology 

LT8410/LT8410-1

APPLICATIONS INFORMATION

Step 4. Calculate the nominal output current:

IOUT(NOM) =

IIN(AVG) • VIN • 0.7 mA

VOUT

Step 5. Derate output current:

IOUT = IOUT(NOM) • 0.8

For low output voltages the output current capability will

be increased. When using output disconnect (load current

taken from VOUT), these higher currents will cause the

drop in the PMOS switch to be higher resulting in lower

output current capability than predicted by the preceding

equations.

Inrush Current

When VCC is stepped from ground to the operating voltage

while the output capacitor is discharged, a high level of

inrush current may flow through the inductor and Schottky

diode into the output capacitor. Conditions that increase

inrush current include a larger more abrupt voltage step

at VCC, a larger output capacitor tied to the CAP pin and

an inductor with a low saturation current. While the chip is

designed to handle such events, the inrush current should

not be allowed to exceed 0.3A. For circuits that use output

capacitor values within the recommended range and have

input voltages of less than 6V, inrush current remains low,

posing no hazard to the device. In cases where there are

large steps at VCC (more than 6V) and/or a large capacitor

is used at the CAP pin, inrush current should be measured

to ensure safe operation.

Soft-Start

The LT8410 series contains a soft-start circuit to limit peak

switch currents during start-up. High start-up current

is inherent in switching regulators in general since the

feedback loop is saturated due to VOUT being far from

its final value. The regulator tries to charge the output

capacitor as quickly as possible, which results in large

peak current.

When the FBP pin voltage is generated by a resistor divider

from the VREF pin, the start-up current can be limited by

connecting an external capacitor (typically 47nF to 220nF)

to the VREF pin. When the part is brought out of shutdown,

10

this capacitor is first discharged for about 70μs (providing

protection against pin glitches and slow ramping), then

an internal 10μA current source pulls the VREF pin slowly

to 1.235V. Since the VOUT voltage is set by the FBP pin

voltage, the VOUT voltage will also slowly increase to the

regulated voltage, which results in lower peak inductor

current. The voltage ramp rate on the pin can be set by

the value of the VREF pin capacitor.

Output Disconnect

The LT8410 series has an output disconnect PMOS that

blocks the load from the input during shutdown. The

maximum current through the PMOS is limited by circuitry

inside the chip, helping the chip survive output shorts.

SHDN Pin Comparator and Hysteresis Current

An internal comparator compares the SHDN pin voltage

with an internal voltage reference (1.3V) which gives a

precise turn-on voltage level. The internal hysteresis of this

turn-on voltage is about 60mV. When the chip is turned on,

and the SHDN pin voltage is close to this turn-on voltage,

0.1μA current flows out of the SHDN pin. This current is

called SHDN pin hysteresis current, and will go away when

the chip is off. By connecting the external resistors as in

Figure 2, a user-programmable enable voltage function

can be realized.

The turn-on voltage for the configuration is:

1.30 • (1 + R1/R2)

and the turn-off voltage is:

(1.24 – R3 • 10–7) • (1 + R1/R2) – (R1 • 10–7)

where R1, R2 and R3 are resistance value in Ω.

ENABLE VOLTAGE

R1

R3

R2

CONNECT TO

SHDN PIN

Figure 2. Programming Enable Voltage by Using External Resistors

84101f

Share Link: