LTC3203_15 Datasheet, PDF (11/16 Pages) Linear Technology – 500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps

LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
APPLICATIO S I FOR ATIO
resistor R_X. Thus, this configuration will have the highest
loop gain giving it the lowest closed-loop output resis-
tance. Likewise it will also require the largest amount of
output capacitance to preserve stability.
Effective Open Loop Output Resistance (R_OL)
The effective open loop output resistance (R_OL) of a
charge pump is a very important parameter, which deter-
mines its strength. The value of this parameter depends on
many factors such as the oscillator frequency (f_OSC), the
value of the flying capacitor (C_FLY), the non-overlap time,
the internal switch resistances (R_S), and the ESR of the
external capacitors.
Maximum Available Output Current
Figure 3 shows how the LTC3203/LTC3203-1/LTC3203B/
LTC3203B-1 can be modeled as a Thevenin-equivalent
circuit.
Thus the maximum available output current and voltage
can be calculated from the effective open-loop output
resistance, R_OL, and the effective output voltage, 1.5V_IN
(in 1.5x mode) or 2V_IN (in 2x mode). From Figure 3, the
available current is given by:
I_OUT
= ^{1.5VIN − VOUT}
R_OL
In
1.5x
mod e
I_OUT
=
2V_IN − V_OUT
R_OL
In
2x
mode
As evident from the above two equations, with the same
V_IN and R_OL, the 2x mode will give more output current
than the 1.5x mode.
1.5V_IN
OR
2V_IN
R_OL
–+
+
V_OUT
–
3203 F03
Figure 3. Charge Pump Open-Loop Thevenin-Equivalent Circuit
Programming the MODE Pin
By connecting a resistor divider to the MODE pin, the V_IN
voltage at which the chip switches modes can be accu-
rately programmed.
When V_IN ramps up, the voltage at the MODE pin crosses
V_MODEH and the chip switches from 2x mode to 1.5x mode.
When V_IN starts to drop, the voltage at the MODE pin
crosses V_MODEL and the chip switches back to 2x mode.
The MODE pin resistor ratio must be selected such that at
the switch point the output is still able to maintain regula-
tion at maximum I_OUT:
1.5 • V_IN(1.5x) – V_OUT > I_OUT • R_OL(1.5X)
The minimum V_IN operating in 1.5x mode occurs at the
switch point where:
V_IN
=
V_MODEL
•
⎛
⎝⎜
R_MODE1
R_MODE2
+
⎞
1_⎠⎟
therefore:
1.5
•
V_MODEL
•
⎛
⎝⎜
R_MODE1
R_MODE2
+
⎞
1_⎠⎟
> R_{OL(1.5X)(MAX)} •I_OUT(MAX) + V_OUT(MIN)
R_{MODE1 >} ^V_OUT(MIN) ^{+ R}_{OL(1.5x)(MAX)} ^•I_{OUT(MAX) – 1}
R_MODE2
1.5 • V_MODEL
V_IN ⁷
LTC3203/
LTC3203B ₆
MODE
9, 11
GND
R_MODE1
C_IN
R_MODE2
3203 F04
Figure 4
32031fa
11