LTC3350_15 Datasheet, PDF (24/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Applications Information
Compensation
The input current, charge current, V_CAP voltage, and V_OUT
voltage loops all require a 1nF to 10nF capacitor from the
VC node to ground. When using the output ideal diode and
backing up to low voltages (<8V) use 8.2nF to 10nF on
VC. When not using the output ideal diode 4.7nF to 10nF
on VC is recommended. For very high backup voltages
(>15V) 1nF to 4.7nF is recommended.
In addition to the VC node capacitor, the V_OUT voltage loop
requires a phase-lead capacitor, C_FBO1, for stability and
improved transient response during input power failure
(Figure 7). The product of the top divider resistor and the
phase-lead capacitor should be used to create a zero at
approximately 2kHz:
R_FBO1
•
C_FBO1 ≈
1
2π(2kHz)
Choose an R_FBO1 such that C_FBO1 is ≥ 100pF to minimize
the effects of parasitic pin capacitance. Because the phase-
lead capacitor introduces a larger ripple at the input of
the V_OUT transconductance amplifier, an additional RC
lowpass filter from the V_OUT divider to the OUTFB pin may
be needed to eliminate voltage ripple spikes. The filter time
constant should be located at the switching frequency of
the synchronous controller:
R_FO
•
C_FO
=
1
2πf_SW
with C_FO > 10pF to minimize the effects of parasitic pin
capacitance. For back up applications where the V_OUT
regulation voltage is low (~5V to 6V), an additional 1k to
3k resistor, R_C, in series with the VC capacitor can improve
stability and transient response.
Minimum V_CAP Voltage in Backup Mode
In backup mode, power is provided to the output from the
supercapacitors either through the output ideal diode or
the synchronous controller operating in step-up mode.
The output ideal diode provides a low loss power path
from the supercapacitors to V_OUT. The minimum internal
(open-circuit) supercapacitor voltage will be equal to
the minimum V_OUT necessary for the system to operate
plus the voltage drops due to the output ideal diode and
equivalent series resistance, R_SC, of each supercapacitor
in the stack.
Example: System needs 5V to run and draws 1A during
backup. There are four supercapacitors in the stack, each
with an R_SC of 45mΩ. The output ideal diode forward
regulation voltage is 30mV (OUTFET R_DS(ON) < 30mΩ).
The minimum open-circuit supercapacitor voltage is:
V_CAP(MIN) = 5V + 0.030V + (1A • 4 • 45mΩ) = 5.21V
Using the synchronous controller in step-up mode allows
the supercapacitors to be discharged to a voltage much
lower than the minimum V_OUT needed to run the system.
The amount of power that the supercapacitor stack can
deliver at its minimum internal (open-circuit) voltage should
be greater than what is needed to power the output and
the step-up converter.
According to the maximum power transfer rule:
P_CAP(MIN)
=
V_CAP(MIN)2
4 •n •R_SC
>
P_BACKUP
η
In the equation above η is the efficiency of the synchro-
nous controller in step-up mode and n is the number of
supercapacitors in the stack.
Example: System needs 5V to run and draws 1A during
backup. There are four supercapacitors in the stack (n = 4),
each with an R_SC of 45mΩ. The converter efficiency is
90%. The minimum open-circuit supercapacitor voltage is:
V_CAP(MIN) =
4 • 4 • 45mΩ • 5V •1A
0.9
= 2.0V
In this case, the voltage seen at the terminals of the ca-
pacitor stack is half this voltage, or 1V, according to the
maximum power transfer rule.
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