SLVS859 – JUNE 2008
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DETAILED DESCRIPTION
V
IN
V
S
EN
IN
FREQ
SW
SS
Current limit
and
Soft Start
Toff Generator
Bias Vref = 1.24 V
UVLO
Thermal Shutdown
Ton
PWM
Generator
Gate Driver of
Power
Transistor
COMP
GM Amplifier
FB
Vref
PGND
Figure 11. Block Diagram
The boost converter is designed for output voltages up to 18.5 V with a switch peak current limit of 2.0 A
minimum. The device, which operates in a current mode scheme with quasi-constant frequency, is externally
compensated for maximum flexibility and stability. The switching frequency is selectable between 650 kHz and
1.2 MHz and the minimum input voltage is 2.3 V. To control the inrush current at start-up a soft-start pin is
available.
During the on-time, the voltage across the inductor causes the current in it to rise. When the current reaches a
threshold value set by the internal GM amplifier, the power transistor is turned off, the energy stored into the
inductor is then released and the current flows through the Schottky diode towards the output of the boost
converter. The off-time is fixed for a certain V
IN
and V
S
, and therefore maintains the same frequency when
varying these parameters.
However, for different output loads, the frequency may slightly change due to the voltage drop across the Rdson
of the power transistor which will have an effect on the voltage across the inductor and thus on t
ON
(t
OFF
remains
fixed). Some slight frequency changes might also appear with a fixed output load due to the fact that the output
voltage V
S
is not sensed directly but via the SW Pin, which affects accuracy.
Because of the quasi-constant frequency behavior of the device, the TPS61085 eliminates the need for an
internal oscillator and slope compensation, which provides better stability for the system over a wide of input and
output voltages range, and more stable and accurate current limiting operation compared to boost converters
operating with a conventional PWM scheme. The TPS61085 topology has also the benefits of providing very
good load and line regulations, and excellent load transient response.
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