AS1336 austriamicrosystems, AS1336 Datasheet - Page 11

• Manufacturer Part Number: AS1336
• Description: Dual, Low Voltage, Micropower Dc-dc Step-up Converters
• Manufacturer: austriamicrosystems

AS1336
Datasheet - D e t a i l e d D e s c r i p t i o n
Modern portable devices frequently spend extended time in low-power or standby modes, switching to high power-
drain only when certain functions are enabled. The AS1336 is ideal for portable devices since it maintain high-power
conversion efficiency over a wide output power range, thus increasing battery life in these types of devices.
In addition to high-efficiency at moderate and heavy loads, the AS1336 includes an automatic powersave mode that
improves efficiency of the power converter at light loads. The powersave mode is initiated if the output load current
falls below a factory programmed threshold.
Low-Voltage Start-Up
The AS1336 requires V
of only 0.8V or higher to start up.
IN
With a V
> 1.6V, the start-up circuitry is disabled and normal fixed-frequency PWM operation is initiated. In this
OUT
mode, the AS1336 operates independent of V
, allowing extended operating time as the battery can drop to several
IN
tenths of a volt without affecting output regulation. The limiting factor for the application is the ability of the battery to
supply sufficient energy to the output.
Low-Noise Fixed-Frequency Operation
Oscillator
The AS1336 switching frequency is internally fixed at 1.2MHz allowing the use of very small external components.
Error Amplifier
The integrated error amplifier is an internally compensated trans-conductance (g
) type (current output). The internal
m
0.8V reference voltage is compared to the voltage at pin FB to generate an error signal at the output of the error ampli-
fier. For the adjustable output voltage version, a voltage divider from V
to GND programs the output voltage from
OUT
1.8 to 3.6V via pin FB as:
V
= 0.8V(1 + (R
/R
))
(EQ 1)
OUT
1
2
Current Sensing
A signal representing the internal NMOS-switch current is summed with the slope compensator. The summed signal is
compared to the error amplifier output to provide a peak current control command for the PWM. Peak switch current is
limited to approximately 500mA independent of V
or V
.
IN
OUT
Zero Current Comparator
The zero current comparator monitors the inductor current to the output and shuts off the PMOS synchronous rectifier
once this current drops to 20mA (approx.). This prevents the inductor current from reversing polarity and results in
improved converter efficiency at light loads.
Anti-Ringing Control
Anti-ringing control circuitry prevents high-frequency ringing on pin SW as the inductor current approaches zero. This
is accomplished by damping the resonant circuit formed by the inductor and the capacitance on pin SW (C
).
SW
Powersave Operation
Powersave operation is enabled for one boost converter if the correspondent MODE pin is set to GND. This setting
applies to both boost converters integrated on the chip. Each boost converter will then switch into powersave mode
independently on the other if a low output current is below the powersave operation current threshold.
In light load conditions, the integrated powersave feature removes power from all circuitry not required to monitor
V
. When V
has dropped approximately 1% from nominal, the AS1336 powers up and begins normal PWM
OUT
OUT
operation.
C
(see Figure 26 on page 10)
recharges, causing the AS1336 to re-enter powersave mode as long as the output
OUT
load remains below the powersave threshold. The frequency of this intermittent PWM is proportional to load current;
i.e., as the load current drops further below the powersave threshold, the AS1336 turns on less frequently. When the
load current increases above the powersave threshold, the AS1336 will resume continuous, seamless PWM operation.
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