lt3689-5 Linear Technology Corporation, lt3689-5 Datasheet - Page 21

• Manufacturer Part Number: lt3689-5
• Description: 700ma Step-down Regulator With Power-on Reset And Watchdog Timer
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Another way to lower the start-up voltage is by using a
resistor divider on the EN/UVLO pin (see the Shutdown
and Undervoltage Lockout section). A resistor divider on
EN/UVLO pin programs the turn-on threshold to slightly
higher than the minimum V
load. Below the EN/UVLO high voltage, the part will stay
shut off and the output cap will remain discharged during
the worse case slow V
crosses the EN/UVLO high threshold, the part will turn on
and the empty output capacitor will provide enough load
to bring the output voltage in regulation. This technique
signifi cantly lowers the start-up voltage of the circuit. The
plot in Figure 7 depicts the minimum load required to start
and run (as a function of input voltage). It also depicts the
benefi t of programming the EN/UVLO threshold to lower
the start-up voltage at low load currents. At light loads, the
inductor current becomes discontinuous and the effective
7.0
6.5
6.0
5.5
5.0
8.0
7.5
1
V
L = 8.2μH
TO RUN
OUT
= 5V
IN
TO START:
EN/UVLO TIED TO V
ramp. When the EN/UVLO pin
IN
f
T
SW
A
voltage required to run at full
10
= 25°C
= 800KHz
LOAD (mA)
TO START: EN/UVLO HIGH
THRESHOLD = 6.15V
Figure 7. LT3689 Minimum V
IN
0.5A/DIV
0.2A/DIV
2V/DIV
100
V
OUT
I
V
V
VIN
IN
OUT
I
L
= 12V
= 5V
3589 F07a
Figure 8. Internal Soft-Start
1000
f = 2MHz
C
IN
= 1μF CERAMIC + 100μF ELECTROLYTIC
20μs/DIV
IN
duty cycle can be very high. This reduces the minimum input
voltage to approximately 300mV above V
currents, the inductor current is continuous and the duty
cycle is limited by the maximum duty cycle of the LT3689,
requiring a higher input voltage to maintain regulation.
Soft-Start
The LT3689 has an internal soft-start that gradually ramps
up the switch current limit from about 100mA to the
switch’s maximum current limit in typical value of 150μs,
as shown in Figure 8. This feature limits the inrush current
during start-up and prevents the switch current from spik-
ing when the EN/UVLO pin crosses the UVLO threshold.
A soft-start sequence is also initiated right after a V
overvoltage/undervoltage lockout, or thermal shutdown
fault in order to prevent the switch current from suddenly
jumping to its maximum current limit.
to Start and Run vs Load
5.0
4.5
4.0
3.5
3.0
6.0
5.5
1
V
L = 8.2μH
TO RUN
OUT
3689 F08
= 3.3V
f
T
10
SW
A
= 25°C
TO START:
EN/UVLO TIED TO V
LOAD (mA)
= 800KHz
TO START: EN/UVLO HIGH
THRESHOLD = 4.85V
LT3689/LT3689-5
100
IN
3589 F07b
1000
OUT
. At higher load
21
3689fa
IN