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LTC3203B-1 Datasheet(PDF) 11 Page - Linear Technology

Part No. LTC3203B-1
Description  500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LTC3203B-1 Datasheet(HTML) 11 Page - Linear Technology

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LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
11
32031fa
resistor RX. 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 (ROL)
The effective open loop output resistance (ROL) 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 (fOSC), the
value of the flying capacitor (CFLY), the non-overlap time,
the internal switch resistances (RS), 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, ROL, and the effective output voltage, 1.5VIN
(in 1.5x mode) or 2VIN (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
VIN and ROL, 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 VIN
voltage at which the chip switches modes can be accu-
rately programmed.
When VIN ramps up, the voltage at the MODE pin crosses
VMODEH and the chip switches from 2x mode to 1.5x mode.
When VIN starts to drop, the voltage at the MODE pin
crosses VMODEL 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 IOUT:
1.5 • VIN(1.5x) – VOUT > IOUT • ROL(1.5X)
The minimum VIN 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
+
⎝⎜
⎠⎟
>
A
AX
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
+
1.5VIN
OR
2VIN
ROL
VOUT
+
APPLICATIO S I FOR ATIO
3203 F04
LTC3203/
LTC3203B
CIN
VIN
MODE
GND
RMODE1
RMODE2
7
6
9, 11
Figure 4


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