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AS1339

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

adjusts for the increase in the load. Before appearing at the PWM comparator, a slope compensation ramp from the

oscillator is subtracted from the error signal for stability of the current feedback loop.

Internal Synchronous Rectifier

To reduce the rectifier forward voltage drop and the associated power loss, the AS1339 uses an internal NFET as a

synchronous rectifier. The big advantage of a synchronous rectification is the higher efficiency in a condition where the

output voltage is low compared to the voltage drop across an ordinary rectifier diode. During the inductor current down

slope in the second part of each cycle the synchronous rectifier is turned on. Before the next cycle the synchronous

rectifier is turned off.

There is no need for an external diode because the NFET is conducting through its intrinsic body diode during the

transient intervals before it turns on.

Bypass Mode

This mode connects IN1A and IN1B directly to PAA and PAB with the internal 110m

Ω (typ) bypass FET, while the step-

down converter is forced into 100% duty-cycle operation during high-power transmission. Due to the low on-resistance

in this mode, the result is low dropout, high efficiency and a high output current capability.

age is limited to 2.78 x 1.5V = 4.17V in Bypass Mode.

Shutdown Mode

To put the PA step-down converter in shutdown mode, connect PA_EN to GND or disconnect PA_EN (NC =>logic-low).

During shutdown mode, the control circuitry, internal switching MOSFET, and synchronous rectifier are turned off and

LX becomes high impedance. For normal operation, connect PA_EN to IN1A/B or logic-high.

To place LDO1 or LDO2 in shutdown mode, connect EN1 or EN2 to GND or disconnect EN1 or EN2 (NC => logic-low).

The outputs of the LDOs are pulled to ground through an internal 100

Ω resistor during shutdown. When the PA step-

down and LDOs are all in shutdown, the AS1339 enters a very low power state, where the input current drops to 0.8μA

(typ).

Note:

All enable Pins (PA_EN, EN1 and EN2) have an internal 110k

Ω pull-down resistance.

Soft-Start

The internal soft-start circuitry of the PA step-down converter limits inrush current at startup, reducing transients on the

input source. Soft-start is favorable for supplies with high output impedance such as Li+ and alkaline cells. The DC-DC

can start-up with full output load of 7.5

Ω.

Analog REFIN Control

The PA step-down converter uses REFIN to set the output voltage, which enables the converter to operate in applica-

tions requiring dynamic voltage control. The output voltage is limited to an upper level of 3.85V, when operating in

Notes:

2. If REFIN is left floating the output voltage of the step-down converter can assume any value between 0.6V and

Thermal Overload Protection

To prevent the AS1339 from short-term misuse and overload conditions the chip includes a thermal overload

protection. To block the normal operation mode the device is turning off the PFET and the NFET in PWM and bypass

mode as soon as the junction temperature exceeds 140°C. To resume the normal operation the temperature has to

drop below 130°C.

Note:

Continuing operation in thermal overload conditions may damage the device and is considered bad practice.