LTC3875
17
3875fb
For more information
www.linear.com/LTC3875
Power Good (PGOOD Pin)
When both V
OSNS
+
pins’ voltages are not within ±7.5% of
the 0.6V reference voltage, the PGOOD pin is pulled low.
The PGOOD pin is also pulled low when the RUN pins are
below 1.14V or when the LTC3875 is in the soft-start,
UVLO or tracking phase. The PGOOD pin will flag power
good immediately when both the V
OSNS
+
pins are within
the ±7.5% of the reference window. However, there is an
internal 20µs power bad mask when V
OSNS
+
voltages go
out of the ±7.5% window. The PGOOD pin is allowed to be
pulled up by an external resistor to sources of up to 6V.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In such cases, the
top MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Fast Transient Operation
The LTC3875 also has a transient improvement function
implemented on
channel 2
. In normal operation, IFAST
pin is floated. This will disable the transient improvement
circuit. To enable the transient improvement function,
connect a resistor from IFAST pin to ground. The voltage
difference between 0.7V and IFAST pin voltage programs
the window of sensitivity of when a transient condition is
detected. During the load step-up, a comparator monitoring
the ripple voltage will compare with the scaled version of
the programmed window voltage and trip. This indicates
that a load step is detected. The LTC3875 will immedi-
ately turn on the top gate and also double the switching
frequency for about 20 cycles.
OPERATION
The plots in Figure 3 show the improvement with and
without the transient improvement circuit for a typical
12V (V
IN
) to 1.5V (V
OUT
) high current application. The
circuit with fast transient shows a near 30% improvement
for the worst case transient steps. For this application,
IFAST pin voltage is programmed to be around 0.62V and
the circuit is not very sensitive to this programmed volt-
age. During the double frequency operation, care has to
be taken not to violate the minimum on-time requirement
of the LTC3875. The fast transient mode is only enabled
in forced continuous mode for channel 2 and is disabled
automatically during start-up, or when output is out of
regulation window.
In order to properly take advantage of the fast transient
circuit, the following equation needs to be satisfied:
V
SENSE(MAX)
30mV
•
0.7 – V
IFAST
25k
+
0.9375
f
OSC
1–
V
OUT
V
IN
• 5k
≥
5•
∆
I
L
•DCR
where,
V
SENSE(MAX)
is the maximum sense threshold voltage
V
IFAST
is the programmed voltage on the IFAST pin
f
OSC
is the programmed switching frequency
V
OUT
is the converter’s output voltage
V
IN
is the converter’s input voltage
ΔI
L
is the inductor ripple current
DCR is the winding resistance of the inductor
As a rule of thumb, the value of the left side of the equa-
tion should be 20% larger than the value of the right side
of the equation.
