as1339 austriamicrosystems, as1339 Datasheet - Page 17

• Manufacturer Part Number: as1339
• Description: 650ma Rf Step-down Dc-dc For Pa, With Two Ldos
• Manufacturer: austriamicrosystems

AS1339
Datasheet - D e t a 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.
The AS1339 enters bypass mode automatically when V
the step-down converter approaches dropout. Due to an internal limitation of V
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
PWM mode. In Bypass mode the output voltage is limited to V
Notes:
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.
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1.
2. If REFIN is left floating the output voltage of the step-down converter can assume any value between 0.6V and
V
V
IN
OUT
.
= 2.5 x V
REFIN
Revision 1.02
IN
≤ 2.69 x V
IN
.
REFIN
and thus prevents excessive output ripple as
REFIN
≤ 1.5V the maximum output volt-
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