t6309a TM Technology Inc., t6309a Datasheet - Page 6

• Manufacturer Part Number: t6309a
• Description: Micropower Step-up Dc/dc Converter
• Manufacturer: TM Technology Inc.

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Functional Description
PFM step-up DC-DC converter, but regulates
the voltage across the sense resistor Rs
instead of the output voltage. This results in a
constant LED current regardless the input
voltage and number of LEDs connected. With
integrated overvoltage protection (OVP), the
T6309A can be used as a current source with
overvoltage protection to drive LEDs. This
allows up to 6 LEDS to be connected in series
to the output. The low 400mV feedback
reference voltage reduces power loss and
improves efficiency.
Step-Up Converter
can be understood by referring to the block
diagram in Figure 1. PFM comparator
monitors the output voltage via the sense
resistor. When the feedback voltage is higher
than the reference voltage, the MOSFET
switch is turned off. As the feedback voltage
is lower than reference voltage and the
MOSFET switch has been off for at least a
period of minimum off-time decided by the
minimum off-time one-shot, the MOSFET
switch is then turned on one-shot, or until the
current limit signal is asserted.
During the internal MOSFET switch is turned
on, current ramps up in the inductor and store
energy in a magnetic field. When the
MOSFET is turned off, the voltage across the
inductor reverses and forces current through
the diode to the output filter capacitor and
load, so the energy in the inductor is
TM Technology Inc. reserves the right
to change products or specifications without notice.
The T6309A is like a standard current-limit
The step-up DC-DC converter operation
CH
TE
P. 6
transferred to output filter capacitor and the
load. As the energy stored in the inductor is
depleted, the current ramps down and the
output diode turns off. The output filter
capacitor stores the charge while the inductor
current is higher than the output current, then
sustains the output voltage until the next
switching cycle.
Current Limit
current limiting by means of protecting the
output MOSFET switch from overstress and
preventing the small value inductor from
saturation. Current limiting is implemented by
monitoring the output MOSFET current
build-up during conduction, and upon sensing
an
turning off the switch for the duration of the
oscillator cycle. The current through the
output MOSFET is monitored and compared
against a reference signal. When the threshold
is reached, a signal is sent to the PFM
controller block to terminate the power switch
conduction. The current limit threshold is
typically set at 300mA.
Overvoltage Protection
source, the output voltage rises as the output
impedance increases or output is open-circuit
(e.g. fault LED).
exceed the 22 V maximum voltage rating of
the internal main switch. An overvoltage
protection circuit is integrated to prevent the
main switch from burning. When the output
Since T6309A is configured as current
The
over-current
T6309A
Publication Date: MAY. 2007
conduction
The output voltage may
utilizes
cycle-by-cycle
T6309A
immediately
Revision: A