AS1345B datasheet(15/29 Pages) AMSCO

15 / 29 page

ams Datasheet, Confidential: 2013-Nov [1-51]

AS1345 – 15

Detaile d Desc rip t ion

Shutdown

Drive EN low to enter shutdown mode. During shutdown the

supply current drops to 0.05μA (typ), the output is

dis-connected from the input, and LX enters a high impedance

state. The capacitance and load at the output set the rate at

which VOUT decays. EN can be pulled as high as 6V regardless

of the input and output voltages.

With a typical step-up converter circuit, the output remains

connected to the input through the inductor and output

rectifier, holding the output voltage to one diode drop below

VDD when the converter is shutdown and allowing the output

to draw power from the input.

The AS1345 features a True-Shutdown mode, disconnecting the

output from the input with an internal P-channel MOSFET

switch when shut down. This eliminates power draw from the

input during shutdown mode.

Start-up and Inrush Limiting

If the ENABLE pin is high, the AS1345 uses a multi-stage start-up

sequence. With increasing supply voltage, first the power-on

circuitry becomes active and some internal blocks are initiated.

If the supply exceeds the under-voltage-lockout threshold

(2.7V typ), the pre-charge-phase is initiated. The capacitor at

the SWOUT pin is charged to VIN, and the capacitor at VOUT is

charged to VIN-VSD. During this phase the current is limited to

100mA typical. After the completion of the pre-charge-phase,

the AS1345 enters into switching mode. Here the specified

current-limit IPEAK is used. The circuit operates at maximum

frequency until the desired VOUT is reached. Then AS1345

switches to normal hysteretic operation mode.

If the load current is too high (>50mA) during the

start-up-phase, the attainment of normal operation mode

might be delayed or not done at all.

Adjustable Output Voltage

The output voltage of the AS1345 is adjustable from 5.0V to 18V

by using a resistor voltage-divider (see Figure 18 and

Figure 19). Select R1 from 10k

Ω to 600kΩ and calculate R2 with

the following equation:

VOUT = VREF (1 + R2/R3)

(EQ1)

Where: VREF = 1.25V

VOUT can range from 5.0V to 18V

For best accuracy, ensure that the bias current through the

feedback resistors is at least 2μA.

The AS1345 can also be used with a fixed output voltage. When

using one of these parts, connect FB directly to the output

(see Figure 20 and Figure 21).

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