LM3670 datasheet(13/25 Pages) TI

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C2 =

2 S R2 10 kHz

* * *

C1 =

2 S R1 10 kHz

* * *

VOUT =

VFB *

+ 1

(

)

VIN,MIN = ILOAD * (RDSON,PFET + RINDUCTOR) + VOUT

LM3670

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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)

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

•

ILOAD is the load current

•

RDSON, PFET is the drain to source resistance of PFET switch in the triode region

•

RINDUCTOR 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 VOUT

to VFB then to GND. VOUT is adjusted to make VFB equal to 0.5V. The resistor from VFB to GND (R2) should be at

least 100K

Ω 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 R2 is 200KΩ, and VFB is 0.5V, then

the current through the resistor feedback network is 2.5µA (IFB =0.5V/R2). The output voltage formula is:

where

•

VOUT Output Voltage (V)

•

VFB Feedback Voltage (0.5V typ)

•

R1 Resistor from VOUT to VFB (Ω)

•

R2 Resistor from VOUT 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 C1 and C2 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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