AS1324 datasheet(15/20 Pages) AMSCO

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AS1324

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

Basic losses in the design of a system should also be considered. Internal battery resistances and copper trace can

account for additional efficiency degradations in battery operated systems. By making sure that CIN has adequate

charge storage and very low ESR at the given switching frequency, the internal battery and fuse resistance losses can

be minimized. CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total

additional loss.

Thermal Shutdown

Due to its high-efficiency design, the AS1324 will not dissipate much heat in most applications. However, in applica-

tions where the AS1324 is running at high ambient temperature, uses a low supply voltage, and runs with high duty

cycles (such as in dropout) the heat dissipated may exceed the maximum junction temperature of the device.

As soon as the junction temperature reaches approximately 150ºC the AS1324 goes in thermal shutdown. In this mode

the internal PMOS & NMOS switch are turned off. The device will power up again, as soon as the temperature falls

below +145°C again.

Checking Transient Response

The main loop response can be evaluated by examining the load transient response. Switching regulators normally

take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an

amount equivalent to:

VDROP =

ΔIOUT x ESR

(EQ 13)

Where:

ESR is the effective series resistance of COUT.

ΔIOUT also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts

to return VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing that

would indicate a stability problem.

Design Example

Figure 28 shows the AS1324 used in a single lithium-ion (3.7V typ) battery-powered mobile phone application. The

load current requirement is 600mA (max) but most of the time the device will require only 2mA (standby mode current).

Figure 28. Design Example

For the circuit shown in Figure 28, efficiency at low- and high-load currents is an important consideration when select-

ing the value for the external inductor, which is calculated as:

From (EQ 14), substituting VOUT = 2.2V, VIN = 3.7V,

ΔIL = 240mA and f = 1.5MHz gives:

Therefore, a standard 2.2µH inductor should be used for this design.

COUT

10µF

CER

CIN

4.7µF

CER

VIN

3.7V

VOUT

2.2V

2.2µH

375k

22pF

AS1324

VIN

VFB

GND

(EQ 14)

OUT

ΔI

--------------

OUT

--------------

–

⎝⎠

⎛⎞

(EQ 15)

2,2V

1,5MHz

240mA

()

----------------------------------------------------

2,2V

37V

------------

–

⎝⎠

⎛⎞

2,48

μH

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