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AS1341

Datasheet - D e t a i l e d D e s c r i p t i o n

8 Detailed Description

The AS1341 step-down converter was specifically designed for battery-powered portable devices, including laptop computers, PDAs, and MP3/

DVD/CD players. The advanced current-limited control scheme provides high-efficiency over a wide range of output loads. The highly-efficient

operation (up to 100% duty cycle) allows extremely low dropout voltage, increasing the usable supply voltage range. In no-load conditions the

AS1341 draws only 12µA; in shutdown mode it draws only 0.8µA to further reduce power consumption and extend battery life.

The AS1341 features an integrated 20V switching MOSFET, internal current sensing, and a high switching frequency, for minimal PCB space

and external component requirements.

Figure 21. AS1341 - Block Diagram - 5V fixed Output Voltage

8.1 Current-Limit Control

The AS1341 uses a proprietary current-limiting control scheme with operation up to 100% duty cycle. The DC-DC converter pulses as needed to

maintain regulation, resulting in a variable switching frequency that increases with the load. This eliminates the high-supply currents associated

with conventional constant-frequency pulse-width-modulation (PWM) controllers that unnecessarily switch the MOSFET.

When the output voltage is too low, the error comparator sets a flip-flop, which turns on the internal P-channel MOSFET and begins a switching

cycle. The inductor current ramps up linearly, storing energy in a magnetic field while charging the output capacitor and servicing the load (see

Figure 19 on page 7).

The MOSFET turns off when the peak current limit is reached, or when the maximum on-time of 10µs is exceeded and the output voltage is in

regulation. If the output is out of regulation and the peak current is never reached, the MOSFET remains on, allowing a duty cycle up to 100%.

This feature ensures the lowest possible dropout voltage.

Once the MOSFET turns off, the flip-flop resets, the inductor current is pulled through D1 (see Figure 21), and the current through the inductor

ramps back down, transferring the stored energy to the output capacitor and load. The MOSFET remains off until the 0.4µs minimum off-time

expires, and the output voltage goes out of regulation.

AS1341

4

LX

8

OUT

5

IN

1

FB

6

ILIMIT

Current Limit

Control

2

GND

+

–

Maximum On-

Time Delay

Q

R

S

Minimum Off-Time

Delay

+

–

+

–

100mV

1.25V

+

–

+

–

+

–

+

–

L1

+

D1

3

POK

7

SHDNN