Datasheet

7
LTC34 06B-2
sn3406b2 3406b2fs
OPERATIO
U
(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.
SUPPLY VOLTAGE (V)
2.5
MAXIMUM OUTPUT CURRENT (mA)
1200
1000
800
600
400
200
0
3.0
3.5 4.0 4.5
3406B F02
5.0 5.5
V
OUT
= 1.8V
V
OUT
= 1.5V
V
OUT
= 2.5V
Figure 2. Maximum Output Current vs Input Voltage
Figure 1a. High Efficiency Step-Down Converter
Figure 1b. Efficiency vs Load Current
V
IN
C
IN
**
4.7µF
CER
V
IN
2.7V
TO 5.5V
*
**
LTC3406B-2
RUN
2.2µH*
3406B F01a
MURATA LQH32CN2R2M33
TAIYO YUDEN JMK212BJ475MG
TAIYO YUDEN JMK316BJ106ML
SW
V
FB
GND
C
OUT
10µF
CER
V
OUT
1.8V
600mA
22pF
1M
499k
OUTPUT CURRENT (mA)
0.1
EFFICIENCY (%)
10
1000
100
90
80
70
60
50
40
30
20
10
3406B F01b
1 100
V
IN
= 2.7V
V
OUT
= 1.8V
T
A
= 25°C
V
IN
= 3.6V
V
IN
= 4.2V
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.