LTC3355_15 Datasheet, PDF (12/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

LTC3355
APPLICATIONS INFORMATION
FB Resistor Network
The V_OUT voltage is programmed with a resistor divider
between the V_OUT pin and the FB pin. Choose the resistor
values according to:
R1= R2^{⎛⎝⎜}
V_OUT
0.8V
– 1^{⎞⎠⎟}
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
CFB Resistor Network
The V_CAP voltage is programmed with a resistor divider
between the V_CAP pin and the CFB pin. Choose the resistor
values according to:
R3
=
R4^{⎛⎝⎜}
V_CAP
0.8V
–
1^{⎞⎠⎟}
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain the capacitor float
voltage accuracy.
I_CHG Set Resistor
The charge current at V_CAP is set by connecting a resistor
from I_CHG to ground. Choose the resistor value according to:
Charger Current (Amps) = ⁶⁰⁴⁰⁰
R5
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain charge current
accuracy.
I_BSTPK Set Resistor
The boost peak current limit is set by connecting a re-
sistor from I_BSTPK to ground. Choose the resistor value
according to:
Boost Peak Current Limit (Amps) = ^1E6
R6
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain boost peak cur-
rent accuracy.
PFI Resistor Network
The V_IN dropout voltage is programmed with a resistor
divider between the V_IN pin and PFI pin. Choose the resis-
tor values according to:
R7
=
R8 ^{⎛⎝⎜}
V_IN
0.8V
–
1^{⎞⎠⎟}
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain the PFI threshold
voltage accuracy.
The V_IN voltage must be greater than the buck dropout
voltage (100% duty cycle) when the PFI level is reached
to ensure that V_OUT stays in regulation.
Input Voltage Range
The minimum input voltage is determined by the dropout
of the buck regulator. The dropout is dependent on the
maximum load current and the buck internal switch resis-
tance. The minimum input voltage due to buck dropout is:
V_IN(MIN) = V_OUT + (I_SW(PEAK) • 1Ω)
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