LTC2927CTS8#PBF Linear Technology, LTC2927CTS8#PBF Datasheet - Page 13

• Manufacturer Part Number: LTC2927CTS8#PBF
• Manufacturer: Linear Technology

Specifications of LTC2927CTS8#PBF

• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Pin Count: 8
• Mounting: Surface Mount
• Package Type: TSOT-23
• Case Length: 2.9mm
• Screening Level: Commercial
• Lead Free Status / RoHS Status: Compliant

APPLICATIO S I FOR ATIO
Final Sanity Checks
The collection of equations below is useful for identifying
unrealizable solutions.
As stated in step 2, the slave supply must fi nish ramping
before the master signal has reached its fi nal voltage. This
can be verifi ed by the following equation:
Here, V
master signal (V
It is possible to choose resistor values that require the
LTC2927 to supply more current than the Electrical Char-
acteristics table guarantees. To avoid this condition, check
that I
exceed ±2mA.
To confi rm that I
guaranteed current, verify that:
Finally, check that the RAMPBUF pin will not be forced to
sink more than 2mA when it is at 0V or be forced to source
more than 2mA when it is at V
ON Pin Resistive Divider
Check that the ON pin voltage is above the 1.25V maximum
threshold at the lowest possible supply voltage value.
Also check that the supply voltage is above the minimum
LTC2927 operating supply voltage of 2.9V before the ON
pin is above the 1.21V minimum threshold voltage.
V
R
R
R
V
V
TRACK
TRACK
ONB
ONA
TA
R
TRACK
TRACK
TB
TRACK
R
<
TB
⎛
⎜
⎝
does not exceed 1mA and I
<
1+
V
< 1
CC MIN
1 25
2
= 0.8V. V
.
mA and
R
R
(
mA
TRACK
CC
TB
TA
V
U
⎞
⎟ <
⎠
if RAMP pin).
)
–
< 1mA, the TRACK pin’s maximum
1
MASTER
V
R
MASTER
V
U
TA
MASTER
+
MASTER
is the fi nal voltage of the
R
TB
W
<
.
2
RAMPBUF
mA
U
does not
For example, if the typical application shown on page 1
has a 3.3V ±10% V
voltage will be 2.97.
If R
Therefore, 138k is chosen. These values must be checked
to ensure the supply reaches the LTC2927 minimum op-
erating supply voltage of 2.9V before the ON pin is above
the minimum threshold.
Load Requirements
When the supply is ramped down quickly, either the load
or the supply itself must be capable of sinking enough
current to support the ramp rate. For example, if there
is a large output capacitance on the supply and a weak
resistive load, supplies that do not sink current will have
their falling ramp rate limited by the RC time constant of
the load and the output capacitance. Figure 17 shows the
case when the slave supply does not track the master
near ground.
1 38
R
R
R
R
ONA
MASTER
.
ONB
ONA
ONB
ONA
SLAVE
<
is 100k then R
<
>
1 21
2 9
.
1 21
.
2 9
2 97
1 25
.
.
.
.
V
V
Figure 17. Weak Resistive Load
V
V
–
V
V
–
IN
1 1 389
–
=
1
, the lowest possible operating supply
1 1 376
=
.
ONB
10ms/DIV
.
must be greater than 137.6k.
LTC2927
2927 F17
1V/DIV
13
2927fb