lt3990 Linear Technology Corporation, lt3990 Datasheet - Page 12

• 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
LT3990
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.

100mA/DIV
5mV/DIV
5V/DIV
V
V
OUT
SW
I
L
FRONT PAGE APPLICATION
V
V
I
LOAD
IN
OUT
Figure 2. Burst Mode Operation
= 12V
= 5V
= 10mA
2µs/DIV
3990 G26
At higher output loads (above ~45mA for the front page
application) the LT3990 will be running at the frequency
programmed by the R
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
Figure 3. Switching Frequency in Burst Mode Operation
700
400
600
500
300
200
100
0
0
FRONT PAGE APPLICATION
50
100
LOAD CURRENT (mA)
T
resistor, and will be operating in
150
200
250
300
3990 F03
350
3990p