Datasheet

LTC3677-3
26
36773f
operaTion
are linked. Therefore, only one of the two trip points can
be chosen, the other is determined by the default ratios
designed in the IC.
Consider an example where a 60°C hot trip point is desired.
From the Vishay curve 1 R-T characteristics, r
HOT
is 0.2488
at 60°C. Using the above equation, R
NOM
should be set
to 46.4k. With this value of R
NOM
, the cold trip point is
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in tem-
perature gain of the thermistor as absolute temperature
increases.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 4. The following formulas can be used
to compute the values of R
NOM
and R1:
R
r r
R
R R r
NOM
COLD HOT
NOM HOT
=
=
–
.
•
. • – •
2 714
25
1 0 536 RR25
For example, to set the trip points to 0°C and 45°C with
a Vishay curve 1 thermistor choose:
R k k
NOM
= =
3 266 0 4368
2 714
100 104 2
. – .
.
• .
the nearest 1% value is 105k.
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
the nearest 1% value is 12.7k. The final solution is shown
in Figure 4 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
Overvoltage Protection (OVP)
The LTC3677-3 can protect itself from the inadvertent
application of excessive voltage to V
BUS
or WALL with
just two external components: an N-channel MOSFET and
a 6.2k resistor. The maximum safe overvoltage magnitude
will be determined by the choice of the external N-channel
MOSFET and its associated drain breakdown voltage.
The overvoltage protection module consists of two pins.
The first, OVSENS, is used to measure the externally applied
voltage through an external resistor. The second, OVGATE,
is an output used to drive the gate pin of an external FET.
The voltage at OVSENS will be lower than the OVP input
voltage by (I
OVSENS
• 6.2kΩ) due to the OVP circuit’s
quiescent current. The OVP input will be 200mV to 400mV
higher than OVSENS under normal operating conditions.
When OVSENS is below 6V, an internal charge pump will
drive OVGATE to approximately 1.88 • OVSENS. This
will enhance the N-channel MOSFET and provide a low
impedance connection to V
BUS
or WALL which will, in
turn, power the LTC3677-3. If OVSENS should rise above
6V (6.35V OVP input) due to a fault or use of an incorrect
wall adapter, OVGATE will be pulled to GND, disabling the
external FET to protect downstream circuitry. When the
voltage drops below 6V again, the external FET will be
re-enabled.
In an overvoltage condition, the OVSENS pin will be
clamped at 6V. The external 6.2k resistor must be
sized appropriately to dissipate the resultant power.
For example, a 1/10W 6.2k resistor can have at most
√P
MAX
• 6.2k = 24V applied across its terminals. With the
6V at OVSENS, the maximum overvoltage magnitude that
this resistor can withstand is 30V. A 1/4W 6.2k resistor
raises this value to 45V.
The charge pump output on OVGATE has limited output
drive capability. Care must be taken to avoid leakage on
this pin, as it may adversely affect operation.