LM3674_15 Datasheet, PDF (12/26 Pages) Texas Instruments – LM3674 2-MHz, 600-mA Step-Down DC-DC Converter in SOT-23

LM3674
SNVS405G – DECEMBER 2005 – REVISED APRIL 2015
8 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Soft-Start
The LM3674 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current
limit is increased in steps. Soft-start is activated only if EN goes from logic low to logic high after V_IN reaches 2.7
V. Soft-start is implemented by increasing switch current limit in steps of 70 mA, 140 mA, 280 mA, and 1020 mA
(typical switch current limit). The start-up time thereby depends on the output capacitor and load current
demanded at start-up. Typical start-up times with 10-µF output capacitor and a 300-mA load current is 350 µs
and with a 10-mA load current is 240 µs.
8.1.2 Low-Dropout (LDO) Operation
The LM3674-ADJ can operate at 100% duty-cycle (no switching, PMOS switch completely on) for low-dropout
support of the output voltage. In this way the output voltage will be controlled down to the lowest possible input
voltage. When the device operates near 100% duty-cycle, the output voltage supply ripple is slightly higher,
approximately 25 mV.
The minimum input voltage needed to support the output voltage is:
V_IN,MIN = I_LOAD × (R_{DSON (P)} + R_INDUCTOR) + V_OUT
where:
• I_LOAD is load current
• R_{DSON (P)} is drain-to-source resistance of PFET switch in the triode region
• R_INDUCTOR is inductor resistance
(3)
8.2 Typical Applications
8.2.1 Typical Application for Fixed Voltage Configuration
V_IN
2.7V to 5.5V
C_IN
4.7 2F
V_IN
1
L1:2.2 2H
SW
5
LM3674
GND
2
V_OUT
C_OUT
10 2F
EN
3
FB
4
Figure 19. Fixed-Voltage Typical Application Circuit
8.2.1.1 Design Requirements
DESIGN PARAMETER
Input voltage
Output voltage
Output current
EXAMPLE VALUE
3.6 V
1.5 V
300 mA
12
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