LTC3200-5_15 Datasheet, PDF (7/12 Pages) Linear Technology – Low Noise, Regulated Charge Pump DC/DC Converters

U
OPERATIO
Maximum Available Output Current
For the adjustable LTC3200, the maximum available out-
put current and voltage can be calculated from the effec-
tive open-loop output resistance, R_OL, and effective output
voltage, 2V_IN(MIN).
R_OL
–+ ^2VIN
+
I_OUT
V_OUT
–
32005 F02
Figure 2. Equivalent Open-Loop Circuit
From Figure 2 the available current is given by:
I_OUT
=
2V_IN – V_OUT
R_OL
Typical R_OL values as a function of temperature are shown
in Figure 3.
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I_OUT = 100mA
C_FLY = 1µF
V_FB = 0V
10
V_IN = 2.7V
V_IN = 3.3V
9
8
–50 –25 0
25 50
75 100
AMBIENT TEMPERATURE (°C)
32005 • F03
Figure 3. Typical R_OL vs Temperature
V_IN, V_OUT Capacitor Selection
The style and value of capacitors used with the LTC3200/
LTC3200-5 determine several important parameters such
as regulator control loop stability, output ripple, charge
pump strength and minimum start-up time.
To reduce noise and ripple, it is recommended that low
ESR (< 0.1Ω) ceramic capacitors be used for both C_IN
and C_OUT. These capacitors should be 0.47µF or greater.
LTC3200/LTC3200-5
Tantalum and aluminum capacitors are not recommended
because of their high ESR.
The value of C_OUT directly controls the amount of output
ripple for a given load current. Increasing the size of C_OUT
will reduce the output ripple at the expense of higher
minimum turn on time and higher start-up current. The
peak-to-peak output ripple is approximately given by the
expression:
V_{RIPPLEP−P}
≅
I_OUT
2f_OSC• C_OUT
Where f_OSC is the LTC3200/LTC3200-5’s oscillator fre-
quency (typically 2MHz) and C_OUT is the output charge
storage capacitor.
Both the style and value of the output capacitor can signifi-
cantly affect the stability of the LTC3200/LTC3200-5. As
shown in the Block Diagrams, the LTC3200/LTC3200-5
use a linear control loop to adjust the strength of the charge
pump to match the current required at the output. The
error signal of this loop is stored directly on the output
charge storage capacitor. The charge storage capacitor
also serves to form the dominant pole for the control loop.
To prevent ringing or instability on the LTC3200-5 it is
important for the output capacitor to maintain at least 0.47µF
of capacitance over all conditions. On the adjustable
LTC3200 the output capacitor should be at least 0.47µF ×
5V/V_OUT to account for the alternate gain factor.
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3200/LTC3200-5.
The closed loop output resistance of the LTC3200-5 is
designed to be 0.5Ω. For a 100mA load current change,
the output voltage will change by about 50mV. If the output
capacitor has 0.3Ω or more of ESR, the closed loop
frequency response will cease to roll off in a simple one
pole fashion and poor load transient response or instabil-
ity could result. Ceramic capacitors typically have excep-
tional ESR performance and combined with a tight board
layout should yield very good stability and load transient
performance.
As the value of C_OUT controls the amount of output
ripple, the value of C_IN controls the amount of ripple
present at the input pin (V_IN). The input current to the
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