LTC3208EUH Linear Technology, LTC3208EUH Datasheet - Page 10

• Manufacturer Part Number: LTC3208EUH
• Description: IC,Laser Diode/LED Driver,CMOS,LLCC,32PIN,PLASTIC
• Manufacturer: Linear Technology

OPERATIO
LTC3208
Soft-Start
Initially, when the part is in shutdown, a weak switch con-
nects V
the CPO output capacitor and prevent large charging
currents to occur.
The LTC3208 also employs a soft-start feature on its charge
pump to prevent excessive inrush current and supply volt-
age droop when switching into the step-up modes. The
current available to the CPO pin is increased linearly over
a typical period of 150µs. Soft start occurs at the start of
both 1.5x and 2x mode changes.
Charge Pump Strength
When the LTC3208 operates in either 1.5x mode or 2x mode,
the charge pump can be modeled as a Thevenin-equivalent
circuit to determine the amount of current available from
the effective input voltage and effective open-loop output
resistance, R
R
switching term, 1/(2f
and the nonoverlap period of the switching circuit. However,
for a given R
directly proportional to the advantage voltage of 1.5V
- CPO for 1.5x mode and 2V
the example of driving white LEDs from a 3.1V supply. If
the LED forward voltage is 3.8V and the current sources
require 100mV, the advantage voltage for 1.5x mode is
3.1V • 1.5 – 3.8V – 0.1V or 750mV. Notice that if the input
voltage is raised to 3.2V, the advantage voltage jumps to
900mV-a 20% improvement in available strength.
From Figure 1, for 1.5x mode the available current is
given by:
10
Figure 1. Charge Pump Thevenin–Equivalent Open-Loop Circuit
OL
I
OUT
is dependent on a number of factors including the
BAT
=
1 5 .
to CPO. This allows V
OL
OL
V
BAT
, the amount of current available will be
(Figure 1).
R
U
OL
+
–
–
OSC
V
1.5V
CPO
• C
BAT
R
BAT
FLY
OL
OR 2V
), internal switch resistances
-CPO for 2x mode. Consider
BAT
BAT1,2
3208 F01
CPO
+
–
to slowly charge
BAT
(1)
For 2X mode, the available current is given by:
Notice that the advantage voltage in the 2x case is
3.1V • 2 – 3.8V – 0.1V = 2.3V. R
significant overall increase in available current is
achieved.
Typical values of R
shown in Figure 2 and Figure 3.
I
OUT
=
Figure 2. Typical 1.5x R
2
Figure 3. Typical 2x R
V
2.5
2.3
2.1
1.9
1.7
1.5
2.8
2.6
2.4
2.2
2.0
1.8
1.6
BAT
–40
–40
R
V
V
C2 = C3 = 2.2µF
C4 = 4.7µF
V
V
C2 = C3 = 2.2µF
C4 = 4.7µF
BAT
CPO
BAT
CPO
OL
–
= 3V
= 3V
= 4.2V
= 4.8V
V
OL
–15
–15
CPO
as a function of temperature are
TEMPERATURE (°C)
TEMPERATURE (°C)
10
10
OL
OL
OL
is higher in 2x mode, but a
35
35
vs Temperature
vs Temperature
60
60
3208 F02
3208 F03
85
85
3208fa
(2)