LTC3255_15 Datasheet, PDF (9/16 Pages) Linear Technology – Wide VIN Range Fault Protected 50mA Step-Down Charge Pump

LTC3255
APPLICATIONS INFORMATION
The LTC3255 has thermal protection that will shut
down the device if the junction temperature exceeds the
overtemperature threshold (typically 175°C). Thermal
shutdown is included to protect the IC in cases of exces-
sively high ambient temperatures, or in cases of excessive
power dissipation inside the IC. The charge transfer will
reactivate once the junction temperature drops back to
approximately 165°C.
When the thermal protection is active, the junction tem-
perature is beyond the specified operating range. Thermal
protection is intended for momentary overload conditions
outside normal operation. Continuous operation above the
specified maximum operating junction temperature may
impair device reliability.
Programming the Output Voltage (FB Pin)
The LTC3255 output voltage is set by connecting its FB
pin to a resistor divider between V_OUT and GND as shown
in Figure 1.
The desired adjustable output voltage is programmed by
solving the following equation for R_A and R_B:
R_{A =} V_{OUT – 1}
R_B 1.2V
Select a value for R_B in the range of 20k to 2M and solve
for R_A. Note that the resistor divider current adds to the
total no-load operating current. Thus a larger value for R_B
will result in lower operating current.
V_OUT
LTC3255
FB
GND
R_A
( ) ^{VOUT =1.2V}
1
+ ^RA
R_B
C_OUT
R_B
3255 F01
Figure 1. Setting the Output Voltage
2:1 Step-Down Charge Pump Operation
In 2:1 step-down mode, charge transfer from V_IN to V_OUT
happens in two phases. On the first phase, the flying ca-
pacitor (C_FLY) is connected between V_IN and V_OUT. On this
phase C_FLY is charged up and current is delivered to V_OUT.
On the second phase, C_FLY is connected between V_OUT and
GND. The charge stored on C_FLY during the first phase is
transferred to V_OUT on the second phase. When in 2:1
step-down mode, the input current will be approximately
half of the total output current. The efficiency (η) and chip
power dissipation (P_D) in 2:1 mode are approximately:
η=
P_OUT
P_IN
=
V_OUT •I_OUT
V_IN • _21IOUT
=
2V_OUT
V_IN
P_D
=


V_IN
2
–
V_OUT


I_OUT
1:1 Step-Down Charge Pump Operation
1:1 step-down mode is similar to how a linear regulator
works. Charge is delivered directly from V_IN to V_OUT through
most of the internal oscillator period. The charge transfer
is briefly interrupted at the end of the period. When in 1:1
step-down mode the input current will be approximately
equal to the total output current. Thus efficiency (η) and
chip power dissipation (P_D) in 1:1 mode are approximately:
η=
P_OUT
P_IN
=
V_OUT •I_OUT
V_IN •I_OUT
=
V_OUT
V_IN
( ) ^P_D ^{= V}_IN ^{– V}_OUT ^I_OUT
3255f
For more information www.linear.com/LTC3255
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