TLV61220
SLVSB53 –MAY 2012
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DETAILED DESCRIPTION
OPERATION
The TLV61220 is a high performance, high efficient boost converter. To achieve high efficiency the power stage
is realized as a synchronous boost topology. For the power switching two actively controlled low RDS(on) power
MOSFETs are implemented.
CONTROLLER CIRCUIT
The device is controlled by a hysteretic current mode controller. This controller regulates the output voltage by
keeping the inductor ripple current constant in the range of 200 mA and adjusting the offset of this inductor
current depending on the output load. In case the required average input current is lower than the average
inductor current defined by this constant ripple the inductor current gets discontinuous to keep the efficiency high
at low load conditions.
IL
Continuous Current Operation
Discontinuous Current Operation
200 mA
(typ.)
200 mA
(typ.)
t
Figure 15. Hysteretic Current Operation
The output voltage VOUT is monitored via the feedback network which is connected to the voltage error amplifier.
To regulate the output voltage, the voltage error amplifier compares this feedback voltage to the internal voltage
reference and adjusts the required offset of the inductor current accordingly. At fixed output voltage versions an
internal feedback network is used to program the output voltage, at adjustable versions an external resistor
divider needs to be connected.
The self oscillating hysteretic current mode architecture is inherently stable and allows fast response to load
variations. It also allows using inductors and capacitors over a wide value range.
Device Enable and Shutdown Mode
The device is enabled when EN is set high and shut down when EN is low. During shutdown, the converter stops
switching and all internal control circuitry is turned off. In this case the input voltage is connected to the output
through the back-gate diode of the rectifying MOSFET. This means that there always will be voltage at the output
which can be as high as the input voltage or lower depending on the load.
Startup
After the EN pin is tied high, the device starts to operate. In case the input voltage is not high enough to supply
the control circuit properly a startup oscillator starts to operate the switches. During this phase the switching
frequency is controlled by the oscillator and the maximum switch current is limited. As soon as the device has
built up the output voltage to about 1.8 V, high enough for supplying the control circuit, the device switches to its
normal hysteretic current mode operation. The startup time depends on input voltage and load current.
Operation at Output Overload
If in normal boost operation the inductor current reaches the internal switch current limit threshold the main
switch is turned off to stop further increase of the input current.
In this case the output voltage will decrease since the device can not provide sufficient power to maintain the set
output voltage.
If the output voltage drops below the input voltage the backgate diode of the rectifying switch gets forward biased
and current starts flow through it. This diode cannot be turned off, so the current finally is only limited by the
remaining DC resistances. As soon as the overload condition is removed, the converter resumes providing the
set output voltage.
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