Datasheet

C
2
=
1
2 S R
2
10 kHz
* * *
C
1
=
1
2 S R
1
10 kHz
* * *
V
OUT
=
R
1
R
2
V
FB
*
+ 1
( )
V
IN,MIN
=
I
LOAD
* (R
DSON,PFET
+ R
INDUCTOR
) + V
OUT
LM3670
www.ti.com SNVS250E –NOVEMBER 2004–REVISED FEBRUARY 2013
Soft-Start
The LM3670 has a soft-start circuit that limits in-rush current during start-up. Typical start-up times with a 10µF
output capacitor and 350mA load is 400µs:
Inrush Current (mA) Duration (µSec)
0 32
70 224
140 256
280 256
620 until soft start ends
LDO - Low Drop Out Operation
The LM3670 can operate at 100% duty cycle (no switching, PMOS switch is completely on) for low drop out
support of the output voltage. In this way the output voltage is controlled down to the lowest possible input
voltage.
The minimum input voltage needed to support the output voltage is
where
• I
LOAD
is the load current
• R
DSON, PFET
is the drain to source resistance of PFET switch in the triode region
• R
INDUCTOR
is the Inductor resistance (5)
Application Information
Output Voltage Selection for Adjustable LM3670
The output voltage of the adjustable parts can be programmed through the resistor network connected from V
OUT
to V
FB
then to GND. V
OUT
is adjusted to make V
FB
equal to 0.5V. The resistor from V
FB
to GND (R2) should be at
least 100KΩ to keep the current sunk through this network well below the 15µA quiescent current level (PFM
mode with no switching) but large enough that it is not susceptible to noise. If R
2
is 200KΩ, and V
FB
is 0.5V, then
the current through the resistor feedback network is 2.5µA (I
FB
=0.5V/R
2
). The output voltage formula is:
where
• V
OUT
Output Voltage (V)
• V
FB
Feedback Voltage (0.5V typ)
• R
1
Resistor from V
OUT
to V
FB
(Ω)
• R
2
Resistor from V
OUT
to GND (Ω) (6)
For any output voltage greater than or equal to 0.7V a frequency zero must be added at 10kHz for stability. The
formula is:
(7)
For any output voltages below 0.7 and above or equal to 2.5V, a pole must also be placed at 10kHz as well. The
lowest output voltage possible is 0.7V. At low output voltages the duty cycle is very small and, as the input
voltage increases, the duty cycle decreases even further. Since the duty cycle is so low any change due to noise
is an appreciable percentage. In other words, it is susceptible to noise. Capacitors C
1
and C
2
act as noise filters
rather than frequency poles and zeros. If the pole and zero are at the same frequency the formula is:
(8)
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