Datasheet

LT3694/LT3694-1
17
36941fb
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
IN
, EN/UVLO
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:
V
IN(UVTRIP)
= 1.2V
R1+ R2
R2






The hysteresis is set by R1:
V
IN(UVHYS)
= 2µA • 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.
APPLICATIONS INFORMATION
Figure 6. Programmable UVLO Application
(6a) EN/UVLO Block Diagram
(6b) Programmable UVLO Application
EN/UVLO
INTERNAL
CIRCUITRY
SHUTDOWN
REGULATORS
0.5V
1.2V
2µA
–
+
–
+
V
IN
V
IN
UVLO HYSTERESIS
2µA • R1
UNDERVOLTAGE TRIP LEVEL
R1
R2
EN/UVLO
1.2V •
(R1 + R2)
R2
LT3694
36941 FO4
The base drive voltage has a maximum voltage of 6V.
This will limit the maximum output of the regulator to
6V – V
BE(SAT)
where V
BE(SAT)
is the base-emitter saturation
voltage of the pass transistor.
Table 5. Low V
CESAT
Transistors
PART NUMBER
V
CESAT
at
I
C
= 1A OUTLINE MANUFACTURER
ZXTN25012EZ
ZXTN25020DG
0.06
0.075
SOT-89
SOT-223
Zetex
www.diodes.com
NSS20201JT1G
NSS12201LT1G
0.22
0.08
SC-89
SOT-23
ON Semiconductor
www.onsemi.com
CTLT3410-M621 0.28
1mm × 2mm
TLM621
Central Semiconductor
www.central-semi.com