LM3670_15 Datasheet, PDF (13/25 Pages) Texas Instruments – LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

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)
0
70
140
280
620
Duration (µSec)
32
224
256
256
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
V_IN,MIN = I_LOAD * (R_DSON,PFET + R_INDUCTOR) + V_OUT
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 R₂ is 200KΩ, and V_FB is 0.5V, then
the current through the resistor feedback network is 2.5µA (I_FB =0.5V/R₂). The output voltage formula is:
V_OUT =
R₁
V_FB * (
+ 1)
R₂
where
• V_OUT Output Voltage (V)
• V_FB Feedback Voltage (0.5V typ)
• R₁ Resistor from V_OUT to V_FB (Ω)
• R₂ 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:
1
C₁ = _{2 *S *R1*10 kHz}
(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:
1
C₂ = _{2 *S *R2*10 kHz}
(8)
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