LTC3210
operation
Soft-Start
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.
Initially, when the part is in shutdown, a weak switch
connects V to CPO. This allows V to slowly charge
BAT
BAT
the CPO output capacitor to prevent large charging
currents.
The LTC3210 also employs a soft-start feature on its
charge pump to prevent excessive inrush current and
supplydroopwhenswitchingintothestep-upmodes. 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.
FromFigure2,for1.5xmodetheavailablecurrentisgivenby:
(1.5VBAT – VCPO
ROL
)
IOUT
=
For 2x mode, the available current is given by:
(2VBAT – VCPO
ROL
)
IOUT
=
Charge Pump Strength and Regulation
Regulation is achieved by sensing the voltage at the CPO
pin and modulating the charge pump strength based
on the error signal. The CPO regulation voltages are set
internally, and are dependent on the charge pump modes
as shown in Table 1.
Notice that the advantage voltage in this case is 3.1V • 2
– 3.8V – 0.1V = 2.3V. R is higher in 2x mode but a sig-
OL
nificant overall increase in available current is achieved.
Typical values of R as a function of temperature are
OL
shown in Figure 3 and Figure 4.
Table 1. Charge Pump Output Regulation Voltages
CHARGE PUMP MODE
REGULATED V
4.55V
CPO
Shutdown Current
1.5x
2x
In shutdown mode all the circuitry is turned off and the
5.05V
LTC3210 draws a very low current from the V supply.
BAT
Furthermore, CPO is weakly connected to V . The
BAT
WhentheLTC3210operatesineither1.5xmodeor2xmode,
thechargepumpcanbemodeledasaThevenin-equivalent
circuit to determine the amount of current available from
the effective input voltage and effective open-loop output
LTC3210 enters shutdown mode when both the ENM
and ENC pins are brought low for 150µs (typ). ENM and
ENC have 250k internal pull down resistors to define
the shutdown state when the drivers are in a high imped-
ance state.
resistance, R (Figure 2).
OL
R
is dependent on a number of factors including the
OL
3.8
V
V
= 3V
BAT
CPO
switchingterm,1/(2f •C ),internalswitchresistances
OSC FLY
= 4.2V
3.6
andthenonoverlapperiodoftheswitchingcircuit.However,
C2 = C3 = C4 = 2.2µF
for a given R , the amount of current available will be
3.4
3.2
3.0
2.8
2.6
2.4
OL
directly proportional to the advantage voltage of 1.5V
BAT
–CPOfor1.5xmodeand2V –CPOfor2xmode.Consider
BAT
R
OL
+
+
CPO
1.5V
OR 2V
BAT
BAT
–
–15
10
35
85
–40
60
–
TEMPERATURE (˚C)
3210 F03
Figure 2. Charge Pump Thevenin-Equivalent Circuit
Figure 3. Typical 1.5x ROL vs Temperature
3210fb
ꢀ0
Linear Systems [ Linear Systems ]
