6
LTC3202
3202fa
Figure 1. Current Control Mode
3202 F01
LTC3202
V
OUT
FB
GND
1µF
• • •
I
LED
=
V
FB
R
X
5, 11
3
2
R
X
R
X
OPERATIO
U
The LTC3202 uses a fractional conversion switched ca-
pacitor charge pump to boost V
OUT
to as much as 1.5 times
the input voltage. A two-phase nonoverlapping clock acti-
vates the charge pump switches. On the first phase of the
clock the flying capacitors are charged in series from V
IN
.
On the second phase of the clock they are connected in
parallel and stacked on top of V
IN
. This sequence of
charging and discharging the flying capacitors continues
at a free running frequency of 1.5MHz (typ).
Regulation is achieved by sensing the voltage at the FB pin
and modulating the charge pump strength based on the
error signal. The control pins, D0 and D1, program the set
point of the internal digital-to-analog converter. The regu-
lation loop will increase V
OUT
until FB comes to balance at
the set-point voltage. Table 1 shows the regulation voltage
as a function of D0 and D1.
Table 1. Feedback Control Voltage Settings
D1 D0 Feedback Set Point Voltage
0 0 Shutdown
0 1 0.2V
1 0 0.4V
1 1 0.6V
In shutdown mode all circuitry is turned off and the
LTC3202 draws only leakage current from the V
IN
supply.
Furthermore, V
OUT
is disconnected from V
IN
. The D0 and
D1 pins are CMOS inputs with a threshold voltage of
approximately 0.8V. The LTC3202 is in shutdown when a
logic low is applied to both D0 and D1. Since the D0 and D1
pins are high impedance CMOS inputs they should never
be allowed to float. To ensure that their states are defined
they must always be driven with valid logic levels.
Shutdown Current
Output voltage detection circuitry will draw a current of
5µA when the LTC3202 is in shutdown. This current will be
eliminated when the output voltage (V
OUT
) is at 0V. To
ensure that V
OUT
is at 0V in shutdown a bleed resistor can
be used from V
OUT
to GND. 10k to 100k is acceptable.
Short-Circuit/Thermal Protection
The LTC3202 has built-in short-circuit current limiting as
well as over temperature protection. During short-circuit
conditions it will automatically limit its output current to
approximately 250mA. At higher temperatures, or if the
input voltage is high enough to cause excessive self
heating on-chip, thermal shutdown circuitry will
shut down the charge pump when the junction tempera-
ture exceeds approximately 160°C. It will reenable the
charge pump once the junction temperature drops back to
approximately 155°C. The LTC3202 will cycle in and out of
thermal shutdown indefinitely without latchup or damage
until the short-circuit on V
OUT
is removed.
Soft-Start
To prevent excessive current flow at V
IN
during start-up,
the LTC3202 has built-in soft-start circuitry. Soft-start is
achieved by increasing the amount of current available to
the output charge storage capacitor linearly over a period
of approximately 500µs.
The soft-start feature activates any time an input, D0 or D1,
changes state. This will prevent large inrush current
during initial start-up as well as when the feedback setting
is changed from one value to the next. Note that the set
point voltage will drop to zero during the soft-start period.
Under heavy load conditions there may be observable
droop at V
OUT
until the soft-start circuit catches up.
Programming the LTC3202 for Voltage or Current
The LTC3202 can be configured to control either a voltage
or a current. In white LED applications the LED current is
programmed by the ratio of the feedback set point voltage
and a sense resistor as shown in Figure 1. The current of
the remaining LEDs is controlled by virtue of their similar-
ity to the reference LED and the ballast voltage across the
sense resistor.
(Refer to Simplified Block Diagram)
