LTC3201 Linear Technology Corporation, LTC3201 Datasheet - Page 5

• Manufacturer Part Number: LTC3201
• Description: 100mA Ultralow Noise Charge Pump Led Supply
• Manufacturer: Linear Technology Corporation

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 0.22 F 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
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
clock. On phase two of the clock, it is stacked in series with
V
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 (<1 A) from the V
supply. Furthermore, V
LTC3201 is in shutdown when a logic low is applied to all
three D0:D2 pins. Note that if V
shutdown current will increase to 10 A 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
to V
inputs, they should never be allowed to float.
Input Current Ripple
The LTC3201 is designed to minimize the current ripple at
V
amounts of current from V
phase 2 of the clocking. If there is a large nonoverlap time
between the two phases, the current being drawn from V
IN
IN
www.DataSheet4U.com
. Typical charge pump boost converters draw large
and connected to V
IN
. Since the D0:D2 pins are high impedance CMOS
U
OUT
OUT
. This sequence of charging and
U
IN
is disconnected from V
IN
than if it is driven all the way
during both phase 1 and
IN
OUT
W
on phase one of the
floats to >1.5V,
OUT
U
and FB.
IN
. The
IN
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
capacitor serves to cancel the higher frequency compo-
nents of the noise, while the V
lower frequency components. The recommended values
are 0.22 F for the FILTER capacitor and 1 F for the V
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
an output LED current that is equal to 0.63V/R
R
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
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.
SENSE
HIGH
HIGH
HIGH
HIGH
LOW
LOW
LOW
LOW
D0
is the resistor connected to any single LED and
F
HIGH
HIGH
HIGH
HIGH
LOW
LOW
LOW
LOW
D1
and the mismatching of the sense
IN
IN
allows the user to program
HIGH
HIGH
HIGH
HIGH
LOW
LOW
LOW
LOW
capacitor cancels out the
D2
LTC3201
IN
pins. The filter
SENSE
Shutdown
0.63V
0.54V
0.45V
0.36V
0.27V
0.18V
0.09V
FB
, where
5
3201f
IN