LTC3255_15 Datasheet, PDF (10/16 Pages) Linear Technology – Wide VIN Range Fault Protected 50mA Step-Down Charge Pump

LTC3255
APPLICATIONS INFORMATION
Power Good Output Operation (PGOOD)
The LTC3255 includes an open-drain power good (PGOOD)
output pin. If the chip is in shutdown or undervoltage
lockout, or if the FB pin voltage is less than 90% (typi-
cal) of its regulation voltage, PGOOD is low impedance
to ground. PGOOD becomes high impedance when V_OUT
rises to 94% (typical) of its regulation voltage. PGOOD
stays high impedance until V_OUT is shut down or drops
below the PGOOD falling threshold (90% typical). A pull-
up resistor can be inserted between PGOOD and V_OUT to
signal a valid power good condition. The use of a large
value pull-up resistor on PGOOD and a capacitor placed
between PGOOD and GND can be used to delay the PGOOD
signal if desired.
V_IN Shunt Regulator Operation
The V_IN shunt regulator feature of the LTC3255 is intended
for applications where V_IN is current-fed, such as in 4mA
to 20mA current loops. A circuit powered by a current
loop must limit the voltage drop it presents to the loop to
avoid exceeding the loop compliance, which would break
the current loop. The LTC3255’s V_IN shunt regulator moni-
tors V_IN and V_OUT, drawing V_IN current as necessary to
keep V_IN from rising much beyond 3V above twice V_OUT.
The shunt regulator is enabled by connecting the SHUNT
pin to the BIAS pin in the circuit board layout. The shunt
is disabled by connecting the SHUNT pin to GND.
The shunt regulator dissipates power which must be ac-
counted for in thermal budgeting. Total power dissipation
(P_DSHUNT) in the LTC3255 with shunt regulator enabled is
equal to the input power minus the output power of the
LTC3255, or approximately:
P_D(SHUNT) = P_IN – P_OUT
= V_IN •I_IN – V_OUT •I_OUT
( ) ^{≈ 2 • V}_OUT ^{+ 2V •I}_IN ^{– V}_OUT ^•I_OUT
where I_IN is the time-averaged current being fed into V_IN
by the current loop, V_OUT is the output voltage, and I_OUT
is the output load current. Notice that the largest power
dissipation occurs when output load current is zero. This
is because any power fed into V_IN must be dissipated in
either the load or the LTC3255. If the load is not drawing
any current, then the LTC3255 must dissipate all of the
input power.
When the shunt regulator is enabled, the LTC3255 charge
pump is locked in 2:1 mode. To achieve output regulation,
the input current to the part must have sufficient voltage
compliance above twice V_OUT. The graph in Figure 2 shows
the typical minimum compliance required at the V_IN pin
for correct operation. For V_OUT ≤ 5.5V, a V_IN compliance
of 2V_OUT + 3.5V is recommended. For V_OUT > 5.5V, a V_IN
compliance of 2V_OUT + 4V is recommended.
3.5
3.4 ^{IVIN = 20mA}
3.3
3.2
3.1
3.0
2.9
2.8
2.7
2.6
2.5
–60 –30
V_OUT = 2.5V
V_OUT = 3.3V
V_OUT = 5V
V_OUT = 12V
0 30 60 90 120 150
TEMPERATURE (°C)
3255 F02
Figure 2. Typical Minimum V_IN – 2 • V_OUT Compliance Required
for Shunt Mode Operation
3255f
10
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