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LTC3200EMS8#PBF

P1-P3P4-P6P7-P9P10-P12
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
LTC3200/LTC3200-5
8
LTC3200/LTC3200-5 will be relatively constant while the
charge pump is on either the input charging phase or the
output charging phase but will drop to zero during the
clock nonoverlap times. Since the nonoverlap time is
small (~25ns), these missing “notches” will result in only
a small perturbation on the input power supply line. Note
that a higher ESR capacitor such as tantalum will have
higher input noise due to the input current change times
the ESR. Therefore ceramic capacitors are again recom-
mended for their exceptional ESR performance.
Further input noise reduction can be achieved by powering
the LTC3200/LTC3200-5 through a very small series in-
ductor as shown in Figure 4. A 10nH inductor will reject the
fast current notches, thereby presenting a nearly constant
current load to the input power supply. For economy the
10nH inductor can be fabricated on the PC board with
about 1cm (0.4") of PC board trace.
R
VV
IfC
OL MIN
IN OUT
OUT OSC FLY
()
–
≡≅
2
1
Where f
OSC
is the switching frequency (2MHz typ) and
C
FLY
is the value of the flying capacitor. The charge pump
will typically be weaker than the theoretical limit due to
additional switch resistance, however for very light load
applications the above expression can be used as a guide-
line in determining a starting capacitor value.
Ceramic Capacitors
Ceramic capacitors of different materials lose their capaci-
tance with higher temperature and voltage at different
rates. For example, a capacitor made of X5R or X7R
material will retain most of its capacitance from – 40°C to
85°C whereas a Z5U or Y5V style capacitor will lose
considerable capacitance over that range. Z5U and Y5V
capacitors may also have a very poor voltage coefficient
causing them to lose 60% or more of their capacitance
when the rated voltage
is applied. Therefore, when com-
paring different capacitors it is often more appropriate to
compare the amount of achievable capacitance for a given
case size rather than discussing the specified capacitance
value. For example, over rated voltage and temperature
conditions, a 1µF, 10V, Y5V ceramic capacitor in an 0603
case may not provide any more capacitance than a
0.22µF, 10V, X7R available in the same 0603 case. In fact
for most LTC3200/LTC3200-5 applications these capaci-
tors can be considered roughly equivalent . The capacitor
manufacturer’s data sheet should be consulted to deter-
mine what value of capacitor is needed to ensure the
desired capacitance at all temperatures and voltages.
Below is a list of ceramic capacitor manufacturers and
how to contact them:
AVX www.avxcorp.com
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay www.vishay.com
OPERATIO
U
Figure 4. 10nH Inductor Used for
Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitor
since its voltage can reverse upon start-up of the LTC3200/
LTC3200-5. Low ESR ceramic capacitors should always
be used for the flying capacitor.
The flying capacitor controls the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have at least 0.68µF of capacitance for the
flying capacitor.
For very light load applications the flying capacitor may be
reduced to save space or cost. The theoretical minimum
output resistance of a voltage doubling charge pump is
given by:
LTC3200/
LTC3200-5
0.22µF
1µF
V
IN
GND
10nH
V
IN
32005 F02
LTC3200/LTC3200-5
9
OPERATIO
U
Power Efficiency
The power efficiency (η) of the LTC3200/LTC3200-5 is
similar to that of a linear regulator with an effective input
voltage of twice the actual input voltage. This occurs
because the input current for a voltage doubling charge
pump is approximately twice the output current. In an ideal
regulating voltage doubler the power efficiency would be
given by:
η≡ = =
P
P
VI
VI
V
V
OUT
IN
OUT OUT
IN OUT
OUT
IN
•
•2 2
At moderate to high output power the switching losses
and quiescent current of the LTC3200/LTC3200-5 are
negligible and the expression above is valid. For example
with V
IN
= 3V, I
OUT
= 50mA and V
OUT
regulating to 5V the
measured efficiency is 80% which is in close agreement
with the theoretical 83.3% calculation.
Operation at V
IN
> 5V
LTC3200/LTC3200-5 will continue to operate with input
voltages somewhat above 5V. However, because of its
constant frequency nature, some charge due to internal
switching will be coupled to V
OUT
causing a slight upward
movement of the output voltage at very light loads. To
avoid an output overvoltage problem with high V
IN
, a
moderate standing load current of 1mA will help the
LTC3200/LTC3200-5 maintain exceptional line regula-
tion. This can be achieved with a 5k resistor from V
OUT
to
GND.
Figure 5. Recommended Layout
Layout Considerations
Due to its high switching frequency and the high transient
currents produced by the LTC3200/LTC3200-5, careful
board layout is necessary. A true ground plane and short
connections to all capacitors will improve performance and
ensure proper regulation under all conditions. Figure 5
shows an example layout for the LTC3200-5.
Thermal Management
For higher input voltages and maximum output current
there can be substantial power dissipation in the LTC3200/
LTC3200-5. If the junction temperature increases above
approximately 160°C the thermal shutdown circuitry will
automatically deactivate the output. To reduce the
maximum junction temperature, a good thermal connec-
tion to the PC board is recommended. Connecting the
GND pin (Pins 4/5 for LTC3200, Pin 2 for LTC3200-5) to
a ground plane, and maintaining a solid ground plane
under the device on two layers of the PC board can reduce
the thermal resistance of the package and PC board
considerably.
Derating Power at Higher Temperatures
To prevent an overtemperature condition in high power
applications Figure 6 should be used to determine the
maximum combination of ambient temperature and power
dissipation.
V
IN
V
OUT
GND
32005 F03
SHDN
1µF 1µF
1µF
LTC3200-5
Figure 6. Maximum Power Dissipation
vs Ambient Temperature
AMBIENT TEMPERATURE (°C)
–50
POWER DISSIPATION (W)
–25
02550
32005 • F06
75 100
θ
JA
= 175°C/W
T
J
= 160°C
1.2
1.0
0.8
0.6
0.4
0.2
0
P1-P3P4-P6P7-P9P10-P12

LTC3200EMS8#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators L N, Reg Ch Pump DC/DC Convs
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