LTC3201_15 Datasheet, PDF (5/8 Pages) Linear Technology – 100mA Ultralow Noise Charge Pump LED Supply with Output Current Adjust

LTC3201
APPLICATIO S I FOR ATIO
Operation (Refer to Simplified Block Diagram)
The LTC3201 is a switched capacitor boost charge pump
especially designed to drive white LEDs in backlighting
applications. The LTC3201’s internal regulation loop
maintains constant LED output current by monitoring the
voltage at the FB pin. The device has a novel internal filter
that, along with an external 0.22µF capacitor, significantly
reduces input current ripple. An internal 7-state DAC
allows the user to lower the regulation voltage at the FB
pin, thus lowering the LED current. To regulate the output
current, the user places a sense resistor between FB and
GND. The white LED is then placed between V_OUT and FB.
The value at the FB pin is then compared to the output of
the DAC. The charge pump output voltage is then changed
to equalize the DAC output and the FB pin. The value of the
sense resistor determines the maximum value of the
output current.
When the charge pump is enabled, a two-phase
nonoverlapping clock activates the charge pump switches.
The flying capacitor is charged to V_IN on phase one of the
clock. On phase two of the clock, it is stacked in series with
V_IN and connected to V_OUT. This sequence of charging and
discharging the flying capacitor continues at a free run-
ning frequency of 1.8MHz (typ) until the FB pin voltage
reaches the value of the DAC.
In shutdown mode all circuitry is turned off and the
LTC3201 draws only leakage current (<1µA) from the V_IN
supply. Furthermore, V_OUT is disconnected from V_IN. The
LTC3201 is in shutdown when a logic low is applied to all
three D0:D2 pins. Note that if V_OUT floats to >1.5V,
shutdown current will increase to 10µA max. In normal
operation, the quiescent supply current of the LTC3201
will be slightly higher if any of the D0:D2 pins is driven high
with a signal that is below V_IN than if it is driven all the way
to V_IN. Since the D0:D2 pins are high impedance CMOS
inputs, they should never be allowed to float.
Input Current Ripple
The LTC3201 is designed to minimize the current ripple at
V_IN. Typical charge pump boost converters draw large
amounts of current from V_IN during both phase 1 and
phase 2 of the clocking. If there is a large nonoverlap time
between the two phases, the current being drawn from V_IN
can go down to zero during this time. At the full load of
100mA at the output, this means that the input could
potentially go from 200mA down to 0mA during the
nonoverlap time. The LTC3201 mitigates this problem by
minimizing the nonoverlap time, using a high (1.8MHz)
frequency clock, and employing a novel noise FILTER
network. The noise filter consists of internal circuitry plus
external capacitors at the FILTER and V_IN pins. The filter
capacitor serves to cancel the higher frequency compo-
nents of the noise, while the V_IN capacitor cancels out the
lower frequency components. The recommended values
are 0.22µF for the FILTER capacitor and 1µF for the V_IN
capacitor. Note that these capacitors must be of the highest
possible resonant frequencies. See Layout Considerations.
3-Bit DAC for Output Current Control
Digital pins D0, D1, D2 are used to control the output
current level. D0 = D1 = D2 = V_IN allows the user to program
an output LED current that is equal to 0.63V/R_SENSE, where
R_SENSE is the resistor connected to any single LED and
connected between FB and ground. Due to the finite
transconductance of the regulation loop, for a given diode
setting, the voltage at the FB Pin will decrease as output
current increases. All LEDs subsequently connected in
parallel should then have similar currents. The mismatch-
ing of the LED V_F and the mismatching of the sense
resistors will cause a differential current error between
LEDs connected to the same output. Once the sense
resistor is selected, the user can then control the voltage
applied across that resistor by changing the digital values
at D0:D2. This in turn controls the current into the LED.
Note that there are only 7 available current states. The 8th
is reserved to shutdown. This is the all 0s code. Refer to
Table below.
D0
HIGH
HIGH
HIGH
HIGH
LOW
LOW
LOW
LOW
D1
HIGH
HIGH
LOW
LOW
HIGH
HIGH
LOW
LOW
D2
HIGH
LOW
HIGH
LOW
HIGH
LOW
HIGH
LOW
FB
0.63V
0.54V
0.45V
0.36V
0.27V
0.18V
0.09V
Shutdown
3201f
5