Datasheet

LTC3200/LTC3200-5
7
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)
.
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
I
fC
RIPPLEP P
OUT
OSC OUT
−
≅
2•
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
OPERATIO
U
From Figure 2 the available current is given by:
I
VV
R
OUT
IN OUT
OL
=
2–
Typical R
OL
values as a function of temperature are shown
in Figure 3.
Figure 2. Equivalent Open-Loop Circuit
+
–
R
OL
I
OUT
V
OUT
2V
IN
32005 F02
+
–
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.
Figure 3. Typical R
OL
vs Temperature
AMBIENT TEMPERATURE (°C)
–50
OUTPUT RESISTANCE (Ω)
–25
02550
32005 • F03
75 100
I
OUT
= 100mA
C
FLY
= 1µF
V
FB
= 0V
11
10
9
8
V
IN
= 3.3V
V
IN
= 2.7V