Datasheet

LT3990/LT3990-3.3/LT3990-5
14
3990fa
the boost capacitor is charged with the energy stored
in the inductor, the circuit will rely on some minimum
load current to get the boost circuit running properly.
This minimum load will depend on input and output volt-
ages, and on the arrangement of the boost circuit. The
minimum load generally goes to zero once the circuit has
started. Figure 5 shows a plot of minimum load to start
and to run as a function of input voltage. In many cases
the discharged output capacitor will present a load to the
switcher, which will allow it to start. The plots show the
worst-case situation where V
IN
is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
V
IN
; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
Enable and Undervoltage Lockout
The LT3990 is in shutdown when the EN/UVLO pin is low
and active when the pin is high. The rising threshold of the
EN/UVLO comparator is 1.19V, with a 35mV hysteresis.
This threshold is accurate when V
IN
is above 4.2V. If V
IN
is lower than 4.2V, tie EN/UVLO pin to GND to place the
part in shutdown.
Figure 6 shows how to add undervoltage lockout (UVLO)
to the LT3990. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
APPLICATIONS INFORMATION
Figure 4. Two Circuits for Generating the Boost Voltage
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where the problems might occur. The UVLO threshold can
be adjusted by setting the values R3 and R4 such that they
satisfy the following equation:
V
UVLO
=
R3
+
R4
R4
• 1.19V
where switching should not start until V
IN
is above V
UVLO
.
Note that due to the comparator’s hysteresis, switching
will not stop until the input falls slightly below V
UVLO
.
Undervoltage lockout is functional only when V
UVLO
is
greater than 5V.
BD
LT3990
(4a) For V
OUT
≥ 2.2V
BOOSTV
IN
V
IN
C3
V
OUT
SW
GND
BD
LT3990
(4b) For V
OUT
< 2.2V; V
IN
< 30V
BOOSTV
IN
V
IN
C3
3990 F04
V
OUT
SW
GND
LOAD CURRENT (mA)
0 50
2.5
INPUT VOLTAGE (V)
3.5
5.0
100
200
250
3.0
4.5
4.0
150
300
350
TO START
TO RUN
FRONT PAGE APPLICATION
V
OUT
= 3.3V
LOAD CURRENT (mA)
0 50
4.0
INPUT VOLTAGE (V)
5.0
6.5
100
200
250
3990 F05
4.5
6.0
5.5
150
300
350
TO START
TO RUN
FRONT PAGE APPLICATION
V
OUT
= 5V, f = 600kHz