LT3970EMS-3.3#PBF Linear Technology, LT3970EMS-3.3#PBF Datasheet - Page 13

• Manufacturer Part Number: LT3970EMS-3.3#PBF
• Description: IC BUCK 3.3V .35A 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3970EMS-3.3#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 3.3V
• Current - Output: 350mA
• Frequency - Switching: 200kHz ~ 2.25MHz
• Voltage - Input: 4.2 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Primary Input Voltage: 12V
• No. Of Outputs: 1
• Output Voltage: 3.3V
• Output Current: 350mA
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
operates at a lower current limit during Burst Mode
operation, 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 fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3970. As pre-
viously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3970 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3970’s rating. This situation
is easily avoided (see the Hot Plugging Safely section).
Low Ripple Burst Mode Operation
To enhance effi ciency at light loads, the LT3970 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 LT3970 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 LT3970 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 LT3970 operates in sleep
mode increases and the average input current is greatly
reduced resulting in high effi ciency 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
3970 F02
At higher output loads (above ~45mA for the front page
application) the LT3970 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 effi ciency. 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 effi cient 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 LT3970 application
is limited by the minimum input voltage (4.2V) and by the
maximum duty cycle as outlined in a previous section. For
output voltages greater than 3.4V, the minimum input volt-
age is also limited by the boost circuit for proper start-up.
If the input voltage is ramped slowly, the boost capacitor
LT3970/LT3970-3.3/LT3970-5
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
3070 F03
350
13
3970fa