AS1321_1 AMSCO [austriamicrosystems AG], AS1321_1 Datasheet - Page 8

• Manufacturer Part Number: AS1321_1
• Description: 130mA Step-Up DC-DC Converter
• Manufacturer: AMSCO [austriamicrosystems AG]

AS1321
Datasheet - D e t a i l e d D e s c r i p t i o n
8 Detailed Description
The AS1321 is a high-efficiency, compact step-up converter with 35µA quiescent supply current which ensures the
highest efficiency over a wide load range. With a minimum of +1.5V input voltage, the device is well suited for applica-
tions with one- or two-cells, such as lithium ion (Li+), nickel-metal-hydride (NiMH), or alkaline.
Figure 17. AS1321 - Block Diagram
The input battery is connected to the device through an inductor and an internal P-FET when pin SHDNN is low. In this
state, the step-up converter is off and the voltage drop across the P-FET body diode is eliminated, and the input bat-
tery can be used as a battery-backup or real-time-clock supply.
The built-in synchronous rectifier significantly improves efficiency.
Control Circuitry
The AS1321 integrated current-limited key circuitry provides low quiescent current and extremely-high efficiency over
a wide V
by the 0.7A N-channel current limit. At each cycle, the inductor current must ramp down to zero after the on-time
before the next cycle may start. When the error comparator senses that the output has fallen below the regulation
threshold, another cycle begins.
Shutdown
When pin SHDNN is low the AS1321 is switched off and no current is drawn from battery; when pin SHDNN is high the
device is switched on. If SHDNN is driven from a logic-level output, the logic high-level (on) should be referenced to
V
Note: If pin SHDNN is not used, it should be connected directly to pin OUT.
In shutdown the battery input is connected to the output through the inductor and the internal synchronous rectifier P-
FET. This allows the input battery to provide backup power for devices such as an idle microcontroller, memory, or real-
time-clock, without the usual diode forward drop. In this way a separate backup battery is not needed.
In cases where there is residual voltage during shutdown, some small amount of energy will be transferred from pin
OUT to pin BATT immediately after shutdown, resulting in a momentary spike of the voltage at pin BATT. The ratio of
C
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+1.5 to +5.0V
OUT
IN
Battery
and C
to avoid intermittently switching the device on.
OUT
22µF
OUT
C
IN
range without the need for an oscillator. Inductor current is limited by the 7µs switch maximum on-time or
partly determine the size and duration of this spike, as does the current-sink ability of the input device.
10µH
SHDNN
BATT
LX
4
2
1
GND
Current
Limiter
3
AS1321
Control
Driver
Logic
and
Revision 1.02
V
Crossing
Detector
REF
Zero
–
+
+1.228V
+1.1V
Circuitry
Startup
+
–
5
OUT
6
RESETN
C
22µF
OUT
+5.0V
Output
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