AS1345 – 14 18V, High Efficiency, DC-DC Step-Up Converter
Modes of Operation
The AS1345 features an advanced current-limited control
scheme operating in hysteretic mode. An internal P-channel
MOSFET switch connects VDD to SWIN to provide power to the
inductor when the converter is operating. When the converter
is shut down, this switch disconnects the input supply from the
inductor (see Figure 3). To boost the output voltage an
N-channel MOSFET switch turns on and allows current to ramp
up in the inductor. Once this current reaches the current limit,
the switch turns off and the inductor current flows through D1
to supply the output. The switching frequency varies
depending on the load and input voltage and can be up to
10kHz.
Shutdown
Drive EN low to enter shutdown mode. During shutdown the
supply current drops to 0.05A (typ), the output is
dis-connected from the input, and LX enters a high impedance
state. The capacitance and load at the output set the rate at
which V
OUT
decays. EN can be pulled as high as 6V regardless
of the input and output voltages.
With a typical step-up converter circuit, the output remains
connected to the input through the inductor and output
rectifier, holding the output voltage to one diode drop below
V
DD
when the converter is shutdown and allowing the output
to draw power from the input.
The AS1345 features a True-Shutdown mode, disconnecting the
output from the input with an internal P-channel MOSFET
switch when shut down. This eliminates power draw from the
input during shutdown mode.
Start-up and Inrush Limiting
If the ENABLE pin is high, the AS1345 uses a multi-stage start-up
sequence. With increasing supply voltage, first the power-on
circuitry becomes active and some internal blocks are initiated.
If the supply exceeds the under-voltage-lockout threshold
(2.7V typ), the pre-charge-phase is initiated. The capacitor at
the SWOUT pin is charged to V
IN
, and the capacitor at V
OUT
is
charged to V
IN
-VSD. During this phase the current is limited to
100mA typical. After the completion of the pre-charge-phase,
the AS1345 enters into switching mode. Here the specified
current-limit I
PEAK
is used. The circuit operates at maximum
frequency until the desired V
OUT
is reached. Then AS1345
switches to normal hysteretic operation mode.
If the load current is too high (>50mA) during the
start-up-phase, the attainment of normal operation mode
might be delayed or not done at all.
