Datasheet

LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
11
32031fa
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
VV
R
In x e
OUT
IN OUT
OL
=
−15
15
.
.mod
I
VV
R
In x e
OUT
IN OUT
OL
=
−2
2mod
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.
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:
VV
R
R
IN MODEL
MODE
MODE
=+
⎛
⎝
⎜
⎞
⎠
⎟
•
1
2
1
therefore:
15 1
1
2
15
.• •
(. )(
V
R
R
R
MODEL
MODE
MODE
OL X M
+
⎛
⎝
⎜
⎞
⎠
⎟
>
AAX OUT MAX OUT MIN
MODE
MODE
OUT M
IV
R
R
V
)() ()
(
• +
>
1
2
IIN OL x MAX OUT MAX
MODEL
RI
V
) (.)() ()
•
.•
–
+
15
15
1
Figure 3. Charge Pump Open-Loop Thevenin-Equivalent Circuit
3203 F03
+
–
1.5V
IN
OR
2V
IN
R
OL
V
OUT
–
+
APPLICATIO S I FOR ATIO
WUUU
3203 F04
LTC3203/
LTC3203B
C
IN
V
IN
MODE
GND
R
MODE1
R
MODE2
7
6
9, 11
Figure 4