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LT3990HMSE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LT3990/LT3990-3.3/LT3990-5
13
3990fa
operates at a lower current limit during Burst Mode op-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3990. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3990 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3990’s rating. This situation
is easily avoided (see the Hot Plugging Safely section).
Low Ripple Burst Mode Operation
To enhance efficiency at light loads, the LT3990 operates
in low ripple Burst Mode operation which keeps the output
capacitor charged to the proper voltage while minimizing
the input quiescent current. During Burst Mode opera-
tion, the LT3990 delivers single cycle bursts of current to
the output capacitor followed by sleep periods where the
output power is delivered to the load by the output capaci-
tor. Because the LT3990 delivers power to the output with
single, low current pulses, the output ripple is kept below
5mV for a typical application. See Figure 2.
As the load current decreases towards a no load condition,
the percentage of time that the LT3990 operates in sleep
mode increases and the average input current is greatly
reduced resulting in high efficiency even at very low loads.
Note that during Burst Mode operation, the switching
frequency will be lower than the programmed switching
frequency. See Figure 3.
At higher output loads (above ~35mA for the front page
application) the LT3990 will be running at the frequency
programmed by the R
T
resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode is seamless, and will not disturb
the output voltage.
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.22µF
capacitor will work well. Figure 4 shows two ways to ar-
range the boost circuit. The BOOST pin must be more than
1.9V above the SW pin for best efficiency. For outputs of
2.2V and above, the standard circuit (Figure 4a) is best.
For outputs between 2.2V and 2.5V, use a 0.47µF boost
capacitor. For output voltages below 2.2V, the boost diode
can be tied to the input (Figure 4b), or to another external
supply greater than 2.2V. However, the circuit in Figure 4a
is more efficient because the BOOST pin current and BD
pin quiescent current come from a lower voltage source.
Also, be sure that the maximum voltage ratings of the
BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3990 application
is limited by the minimum input voltage (4.2V) and by the
maximum duty cycle as outlined in a previous section. For
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
the boost capacitor may not be fully charged. Because
APPLICATIONS INFORMATION
Figure 2. Burst Mode Operation
Figure 3. Switching Frequency in Burst Mode Operation
LOAD CURRENT (mA)
0 50
0
SWITCHING FREQUENCY (kHz)
200
500
100
200
250
3990 F03
100
400
300
150
300
350
FRONT PAGE APPLICATION
V
OUT
5mV/DIV
V
SW
5V/DIV
I
L
100mA/DIV
2µs/DIV
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 10mA
f = 600kHz
3990 G28
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
• 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
LT3990/LT3990-3.3/LT3990-5
15
3990fa
APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, a LT3990 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3990 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3990’s
output. If the V
IN
pin is allowed to float and the EN/UVLO
pin is held high (either by a logic signal or because it is
tied to V
IN
), then the LT3990’s internal circuitry will pull
its quiescent current through its SW pin. This is fine if the
system can tolerate a few µA in this state. If the EN/UVLO
pin is grounded, the SW pin current will drop to 0.7µA.
However, if the V
IN
pin is grounded while the output is held
high, regardless of EN/UVLO, parasitic diodes inside the
LT3990 can pull current from the output through the SW
pin and the V
IN
pin. Figure 7 shows a circuit that will run
only when the input voltage is present and that protects
against a shorted or reversed input.
PCB Layout
For proper operation and minimum EMI, care must
be taken during printed circuit board layout. Figure 8
shows the recommended component placement with
trace, ground plane and via locations. Note that large,
switched currents flow in the LT3990’s V
IN
and SW pins,
the internal catch diode and the input capacitor. The loop
formed by these components should be as small as pos-
sible. These components, along with the inductor and
Figure 7. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output. It Also Protects the Circuit from
a Reversed Input. The LT3990 Runs Only when the Input Is Present
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
Figure 6. Undervoltage Lockoout
+
–
1.19V
SHDN
3990 F06
LT3990
EN/UVLO
V
IN
V
IN
R3
R4
BD
LT3990
BOOSTV
IN
EN/UVLO
V
IN
V
OUT
BACKUP
3990 F07
SW
D4
FBGND
+
6
8
7
9
10
5
4
2
3
1
VIAS TO LOCAL GROUND PLANE
VIAS TO V
OUT
EN/UVLO
GND
GND
PG
V
OUT
GND
V
IN
3990 F08
output capacitor, should be placed on the same side of
the circuit board, and their connections should be made
on that layer. Place a local, unbroken ground plane below
these components. The SW and BOOST nodes should be
as small as possible. Finally, keep the FB nodes small so
that the ground traces will shield them from the SW and
BOOST nodes. The exposed pad on the bottom must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3990 to additional ground planes within the circuit
board and on the bottom side.
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LT3990HMSE#PBF

Mfr. #:
Buy LT3990HMSE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 60V, 350mA Step-Down Regulator with 2uA Quiescent Current and Integrated Diodes in DFN
Lifecycle:
New from this manufacturer.
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