lt3990 Linear Technology Corporation, lt3990 Datasheet - Page 13

• Manufacturer Part Number: lt3990
• Description: 60v, 350ma Step-down Regulator With 2.5?a Quiescent Current And Integrated Diodes
• Manufacturer: Linear Technology Corporation

applicaTions inFormaTion
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
For lower start-up voltage, the boost diode can be tied to
V
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 1.21V, with a 32mV hysteresis.
This threshold is accurate when V
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
IN
; however, this restricts the input range to one-half of
Figure 4. Two Circuits for Generating the Boost Voltage
V
V
IN
IN
(4b) For V
V
V
IN
IN
(4a) For V
LT3990
LT3990
GND
GND
BD
BD
OUT
BOOST
BOOST
SW
SW
< 2.2V; V
OUT
≥ 2.2V
IN
is ramping very slowly.
IN
C3
C3
IN
< 27V
is above 4.2V. If V
3990 F04
V
OUT
V
OUT
IN
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:
where switching should not start until V
Note that due to the comparator’s hysteresis, switching
will not stop until the input falls slightly below V
Undervoltage lockout is functional only when V
greater than 5V.
V
UVLO
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
=
4.0
2.5
5.0
4.5
4.0
3.5
3.0
2.5
5.0
4.5
3.5
3.0
R
0
0
3
FRONT PAGE APPLICATION
f = 600kHz
FRONT PAGE APPLICATION
V
R
OUT
TO START
+
4
R
50
50
= 3.3V
4
TO START
• .
100
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
1 21
TO RUN
150
150
TO RUN
V
200
200
250
250
300
300
IN
3990 F05
is above V
350
350
LT3990

UVLO
UVLO
UVLO
3990p
is
.
.