LTC3217_15 Datasheet, PDF (7/12 Pages) Linear Technology – 600mA Low Noise Multi-LED Camera Light Charge Pumps

LTC3217
OPERATION
The LED currents are delivered by the four programmable
current sources. Three discrete current settings (Low, High,
Low + High) are available and may be selected via the EN1
and EN2 pins. The values of these currents may be selected
by choosing the appropriate programming resistors. Each
resistor is connected between the I_SET1 or I_SET2 pin to
ground. The resistor values required to attain the desired
current levels can be determined by Equation 1.
R_SET1/2
=
488
I_LEDx
(1)
An R_SETx resistor value of 2k or less (i.e., short-circuit) will
cause the LTC3217 to enter overcurrent shutdown mode.
This mode prevents damage to the part and external LEDs
by shutting down the high power sections of the part.
Each LED output can be disabled by connecting the pin
directly to CPO. Do not leave pins open as this will cause
dropout and subsequently mode changing.
Pulse Width Modulation Option
EN2 can be pulse width modulated to control the
LED brightness. The minimum allowable pulse width is
50μs and the maximum low time is 1ms. Pulse width
modulating the EN2 input can be performed with EN1 high
or low. If EN1 is high then there is no limitation on the
EN2 low time. When EN1 is low the part would normally
go into shutdown whenever EN2 goes low. Prevention
of shutdown in this case is achieved by an internal timer
which delays shutdown until EN2 has remained low for
at least 1ms.
Soft-Start
Initially, when the part is in shutdown, a weak switch
connects V_BAT to CPO. This allows V_BAT to slowly charge
the CPO output capacitor and prevent large charging
currents to occur.
The LTC3217 also employs a soft-start feature on its
charge pump to prevent excessive inrush current and
supply droop when switching into the step-up modes. The
current available to the CPO pin is increased linearly over
a typical period of 125μs. Soft-start occurs at the start of
both 1.5x and 2x mode changes.
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.
When the LTC3217 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_OL (Figure 1).
Table 1. Charge Pump Output Regulation Voltages
CHARGE PUMP MODE
1.5x
REGULATED V_CPO
4.5V
2x
5.05V
R_OL is dependent on a number of factors including the
switching term, 1/(2f_OSC • C_FLY), internal switch resistances
and the non-overlap period of the switching circuit. However,
for a given R_OL, the amount of current available will be directly
proportional to the advantage voltage of 1.5V_BAT – V_CPO for
1.5x mode and 2V_BAT – V_CPO for 2x mode. Consider 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:
I_OUT
=
1.5V_BAT –
R_OL
V_CPO
(2)
For 2x mode, the available current is given by:
I_OUT
=
2V_BAT – V_CPO
R_OL
(3)
Notice that the advantage voltage in this case is
3.1V • 2 – 3.8V – 0.1V = 2.3V. R_OL is higher in 2x mode
but a significant overall increase in available current is
achieved.
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