LTC3108 Linear Technology, LTC3108 Datasheet - Page 13

• Manufacturer Part Number: LTC3108
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
COMPONENT SELECTION
Step-Up Transformer
The step-up transformer turns ratio will determine how
low the input voltage can be for the converter to start.
Using a 1:100 ratio can yield start-up voltages as low as
20mV. Other factors that affect performance are the DC
resistance of the transformer windings and the inductance
of the windings. Higher DC resistance will result in lower
effi ciency. The secondary winding inductance will deter-
mine the resonant frequency of the oscillator, according
to the following formula.
Where L is the inductance of the transformer secondary
winding and C is the load capacitance on the secondary
winding. This is comprised of the input capacitance at pin
C2, typically 30pF , in parallel with the transformer secondary
winding’s shunt capacitance. The recommended resonant
frequency is in the range of 20kHz to 200kHz. See Table 4
for some recommended transformers.
Table 4. Recommended Transformers
VENDOR
Coilcraft
www.coilcraft.com
C1 Capacitor
The charge pump capacitor that is connected from the
transformer’s secondary winding to the C1 pin has an ef-
fect on converter input resistance and maximum output
current capability. Generally, a minimum value of 1nF is
recommended when operating from very low input volt-
ages using a transformer with a ratio of 1:100. Too large
a capacitor value can compromise performance when
operating at low input voltage or with high resistance
sources. For higher input voltages and lower turns ratios,
the value of the C1 capacitor can be increased for higher
Frequency
=
2 • •
π
L
PART NUMBER
LPR6235-752SML (1:100 Ratio)
LPR6235-253PML (1:20 Ratio)
LPR6235-123QML (1:50 Ratio)
1
(sec)•
C
Hz
output current capability. Refer to the Typical Applications
schematic examples for the recommended value for a
given turns ratio.
V
For pulsed load applications, the V
be sized to provide the necessary current when the load
is pulsed on. The capacitor value required will be dictated
by the load current, the duration of the load pulse, and
the amount of voltage droop the circuit can tolerate. The
capacitor must be rated for whatever voltage has been
selected for V
Note that there must be enough energy available from
the input voltage source for V
during the interval between load pulses (to be discussed
in the next example). Reducing the duty cycle of the load
pulse will allow operation with less input energy.
The VSTORE capacitor may be of very large value (thou-
sands of microfarads or even Farads), to provide holdup
at times when the input power may be lost. Note that this
capacitor can charge all the way to 5.25V (regardless of
the settings for V
has a working voltage rating of at least 5.5V at the tem-
perature for which it will be used. The VSTORE capacitor
can be sized using the following:
Where 6μA is the quiescent current of the LTC3108, I
the load on V
LDO between bursts, I
burst, t is the duration of the burst, f is the frequency of
the bursts, TSTORE is the storage time required and V
is the output voltage required. Note that for a programmed
output voltage of 5V, the VSTORE capacitor cannot provide
any benefi cial storage time.
OUT
C
C
OUT
STORE
and VSTORE Capacitor
(
mF
≥
)
OUT
⎡ ⎣
≥
OUT
6
µA I
I
OUT
in between bursts, I
LOAD
by VS1 and VS2.
+
), so ensure that the holdup capacitor
Q
(
mA
+
BURST
I
Δ
LDO
) •
V
OUT
5
t
OUT
PULSE
+
. . 25 − V
is the total load during the
(
I
BURST
to recharge the capacitor
(sec)
OUT
LDO
OUT
• • ) •
LTC3108
capacitor should
t f
is the load on the
⎤ ⎦
TSTORE
13
3108p
Q
OUT
is