LTC3203 LINER [Linear Technology], LTC3203 Datasheet - Page 11

• Manufacturer Part Number: LTC3203
• Description: 500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
resistor R
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
The effective open loop output resistance (R
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
value of the flying capacitor (C
the internal switch resistances (R
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
(in 1.5x mode) or 2V
available current is given by:
As evident from the above two equations, with the same
V
than the 1.5x mode.
Figure 3. Charge Pump Open-Loop Thevenin-Equivalent Circuit
IN
I
I
OUT
OUT
and R
=
=
1 5
2
X
OL
.
V
. Thus, this configuration will have the highest
IN
, the 2x mode will give more output current
V
R
OL
IN
R
−
OL
OL
, and the effective output voltage, 1.5V
−
V
OUT
1.5V
V
2V
OUT
U
OR
IN
IN
IN
In x
In
(in 2x mode). From Figure 3, the
2 mod
+
–
U
1 5
.
R
OL
x
FLY
mod
e
), the non-overlap time,
S
W
e
), and the ESR of the
3203 F03
V
OUT
+
–
OL
)
U
OL
OSC
) of a
), the
IN
Programming the MODE Pin
By connecting a resistor divider to the MODE pin, the V
voltage at which the chip switches modes can be accu-
rately programmed.
When V
V
When V
crosses V
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
The minimum V
switch point where:
therefore:
MODEH
1.5 • V
1 5
V
>
R
R
IN
. •
MODE
R
MODE
OL
=
IN
and the chip switches from 2x mode to 1.5x mode.
V
IN
V
( .
MODEL
IN(1.5x)
1 5
MODEL
MODEL
2
ramps up, the voltage at the MODE pin crosses
1
starts to drop, the voltage at the MODE pin
LTC3203B/LTC3203B-1
>
X M
)(
V
OUT M
LTC3203/LTC3203-1
IN
•
A A X
and the chip switches back to 2x mode.
– V
•
LTC3203B
LTC3203/
⎛
⎝ ⎜
⎛
⎝ ⎜
OUT
operating in 1.5x mode occurs at the
R
)
(
R
R
R
MODE
OUT
•
MODE
MODE
I
I I N
GND
MODE
MODE
OUT MAX
:
V
IN
)
> I
+
Figure 4
7
6
9, 11
(
1
2
R
2
1
1 5
OUT
+
OL
. •
+
1
( . )(
1
)
⎞
⎠ ⎟
1 5
• R
⎞
⎠ ⎟
+
V
R
R
MODE1
MODE2
MODEL
V
x MAX
OL(1.5X)
3203 F04
OUT MIN
(
C
)
IN
•
I
)
OUT MAX
(
11
)
32031fa
–
1
IN