www.ams.com/DC-DC_Step-Down

Revision 1.10

10 - 16

AS1346-49

Datasheet - Detailed Description

8.1 Main Control Loop

During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch. This switch is turned off

increase until the average inductor current matches the new load current.

When the top MOSFET is off, the bottom MOSFET is turned on until the inductor current starts to reverse as indicated by the current reversal

comparator, or the next clock cycle begins. The over-voltage detection comparator guards against transient overshoots >7.8% by turning the

main switch off and keeping it off until the transient is removed.

8.2 Short-Circuit Protection

The short-circuit protection turns off the power switches as long as the short is applied. When the short is removed the device is continuing

normal operation.

8.3 Dropout Operation

The AS1346, AS1347, AS1348, AS1349 is working with a low input-to-output voltage difference by operating at 100% duty cycle. In this state,

the PMOS is always on. This is particularly useful in battery-powered applications with a 3.3V output.

The AS1346, AS1347, AS1348, AS1349 allows the output to follow the input battery voltage as it drops below the regulation voltage. The

quiescent current in this state is reduced to a minimal value, which aids in extending battery life. This dropout (100% duty-cycle) operation

achieves long battery life by taking full advantage of the entire battery range.

The input voltage requires maintaining regulation and is a function of the output voltage and the load. The difference between the minimum input

voltage and the output voltage is called the dropout voltage. The dropout voltage is therefore a function of the on-resistance of the internal PMOS

tion should be taken in consideration.

8.4 Shutdown

Connecting EN to GND or logic low places the AS1346, AS1347, AS1348, AS1349 in shutdown mode and reduces the supply current to 0.1μA.

In shutdown the control circuitry and the internal NMOS and PMOS turn off and SW becomes high impedance disconnecting the input from the

Note: Pin EN should not be left floating.

8.4.1 Power-OK and Low-Battery-Detect Functionality

LBO goes low in startup mode as well as during normal operation if,

- The voltage at the LBI pin is above LBI threshold (1.2V). This can be used to monitor the battery voltage.

The LBI pin can be connected to a resistive-divider to monitor a particular definable voltage and compare it with a 1.2V internal reference. If LBI

is connected to GND (see Figure 1 on page 1) an internal resistive-divider is activated and connected to the output. Therefore, the Power-OK

functionality can be realized with no additional external components.

Larger values for this resistor will help to minimize current consumption; a 100k

 resistor is perfect for most applications (see Figure 18 on page

11).

For the circuit shown in the left of Figure 18 on page 11, the input bias current into LBI is very low, permitting large-value resistor-divider networks

Where:

In case of the LBI pin is connected to GND, an internal resistor-divider network is activated and compares the output voltage with a 92.5%

voltage threshold (see AS1346 - Typical Application Diagram with POK Function on page 1). For this particular Power-OK application, no

external resistive components (R1 and R2) are necessary.

(EQ 1)

------------ 1

–







=