Page 12 ams Datasheet
Document Feedback [v1-11] 2015-Jan-28
AS1310 − Detailed Description
Hysteretic Boost Converter
Hysteretic boost converters are so called because comparators
are the active elements used to determine ON-OFF timing via
current and voltage measurements. There is no continuously
operating fixed oscillator, providing an independent timing
reference. As a result, a hysteretic or comparator based
converter has a very low quiescent current. In addition, because
there is no fixed timing reference, the operating frequency is
determined by external component (inductor and capacitors)
and also the loading on the output.
Ripple at the output is an essential operating component. A
power cycle is initiated when the output regulated voltage
drops below the nominal value of V
OUT
(0.99 x V
OUT
).
Inductor current is monitored by the control loop, ensuring that
operation is always dis-continuous.
The application circuit shown in Figure 2 will support many
requirements. However, further optimization may be useful,
and the following is offered as a guide to changing the passive
components to more closely match the end requirement.
Input Loop Timing
The input loop consists of the source DC supply, the input
capacitor, the main inductor, and the N-channel power switch.
The ON timing of the N-channel switch is determined by a peak
current measurement or a maximum ON time. In the AS1310,
peak current is 400mA (typ) and maximum ON time is 4.2μs
(typ). Peak current measurement ensures that the ON time
varies as the input voltage varies. This imparts line regulation
to the converter.
The fixed ON-time measurement is something of a safety
feature to ensure that the power switch is never permanently
ON. The fixed on-time is independent of input voltage changes.
As a result, no line regulation exists.
