LTC3406B-2 Datasheet, PDF (7/16 Pages) Linear Technology – 2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

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OPERATIO (Refer to Functional Diagram)
frequency. This frequency foldback ensures that the in-
ductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will progressively
increase to 2.25MHz when V_FB rises above 0V.
Dropout Operation
As the input supply voltage decreases to a value approach-
ing the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will then
be determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
V_IN
2.7V
TO 5.5V
C_IN**
4.7µF
CER
V_IN
SW
LTC3406B-2
RUN
V_FB
GND
2.2µH*
22pF
1M
C_OUT^†
10µF
CER
V_OUT
1.8V
600mA
499k
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JMK212BJ475MG
†_{TAIYO YUDEN JMK316BJ106ML}
3406B F01a
Figure 1a. High Efficiency Step-Down Converter
100
V_OUT = 1.8V
90 ^TA ^{= 25°C}
80
70
60
V_IN = 3.6V
50
V_IN = 2.7V
40
30
20
10
0.1
V_IN = 4.2V
1
10
100
OUTPUT CURRENT (mA)
1000
3406B F01b
Figure 1b. Efficiency vs Load Current
LTC3406B-2
An important detail to remember is that at low input supply
voltages, the R_DS(ON) of the P-channel switch increases
(see Typical Performance Characteristics). Therefore, the
user should calculate the power dissipation when the
LTC3406B-2 is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Low Supply Operation
The LTC3406B-2 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles > 40%. However, the LTC3406B-2 uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
1200
1000
V_OUT = 1.8V
800
V_OUT = 1.5V
600
V_OUT = 2.5V
400
200
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
3406B F02
Figure 2. Maximum Output Current vs Input Voltage
sn3406b2 3406b2fs
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