LT3694 LINER [Linear Technology], LT3694 Datasheet - Page 17

• Manufacturer Part Number: LT3694
• Description: 36V, 2.6A Monolithic Buck Regulator With Dual LDO
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Once EN/UVLO climbs above the first threshold, the inter-
nal circuitry of the LT3694 is turned on but the switching
regulator and LDOs remain shut off. A 2μA current sink
on the EN/UVLO pin is activated to provide hysteresis for
the programmable undervoltage function.
The second threshold is an accurate 1.2V derived from the
internal reference. When EN/UVLO is above the second
threshold, the regulators turn on and the 2μA current sink
turns off. This allows an accurate programmable UVLO
function by placing a resistor divider between V
and ground. Figure 6a shows the EN/UVLO block diagram
and Figure 6b shows connections for the programmable
UVLO function.
The trip level is set by the resistor ratio:
The hysteresis is set by R1:
The EN/UVLO pin may be driven with a logic output if the
programmable UVLO is not needed. The requirements for
the logic output are a low output voltage less than 0.35V
(to insure low current shutdown) and a high output volt-
age greater than 1.25V.
Low Dropout Regulator
Each low dropout regulator comprises an error amp, loop
compensation and a base drive amp. It uses the same
0.75V reference as the switching regulators. It requires
an external NPN pass transistor and 2.2μF of output ca-
pacitance for stability.
The dropout characteristics will be determined by the pass
transistor. The collector-emitter saturation characteristics
will limit the dropout voltage. Table 5 lists some suitable
NPN transistors with their saturation specifications.
V
V
IN(UVHYS)
IN(UVTRIP)
= 2µA • R1
= 1.2V
⎛
⎝ ⎜
R1+ R2
R2
⎞
⎠ ⎟
IN
, EN/UVLO
The base drive voltage has a maximum voltage of 6V.
This will limit the maximum output of the regulator to
6V – V
voltage of the pass transistor.
Table 5. Low V
PART NUMBER
ZXTN25012EZ
ZXTN25020DG
NSS20201JT1G
NSS12201LT1G
CTLT3410-M621
EN/UVLO
BE(SAT)
V
IN
2μA
Figure 6. Programmable UVLO Application
(6b) Programmable UVLO Application
CESAT
where V
R1
R2
(6a) EN/UVLO Block Diagram
V
I
CESAT
Transistors
C
0.075
0.06
0.22
0.08
0.28
= 1A
V
EN/UVLO
0.5V
1.2V
IN
BE(SAT)
LT3694
at
LT3694/LT3694-1
1mm × 2mm
+
–
+
–
OUTLINE
SOT-223
is the base-emitter saturation
TLM621
SOT-89
SOT-23
SC-89
UNDERVOLTAGE TRIP LEVEL
UVLO HYSTERESIS
1.2V •
2μA • R1
Zetex
www.diodes.com
ON Semiconductor
www.onsemi.com
Central Semiconductor
www.central-semi.com
(R1 + R2)
MANUFACTURER
R2
INTERNAL
CIRCUITRY
SHUTDOWN
REGULATORS
36941 FO4
17
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