ZHCSA01H October 2011 – June 2019 LMR24210
PRODUCTION DATA.
- 1 特性
- 2 應用
- 3 說明
- 4 修訂歷史記錄
- 5 Pin Configuration and Functions
- 6 Specifications
-
7 Detailed Description
- 7.1 Overview
- 7.2 Functional Block Diagram
- 7.3
Feature Description
- 7.3.1 COT Control Circuit Overview
- 7.3.2 Start-up Regulator (VCC)
- 7.3.3 Regulation Comparator
- 7.3.4 Zero Coil Current Detect
- 7.3.5 Overvoltage Comparator
- 7.3.6 On-Time Timer, Shutdown
- 7.3.7 Current Limit
- 7.3.8 N-Channel Mosfet and Driver
- 7.3.9 Soft Start
- 7.3.10 Thermal Protection
- 7.3.11 Thermal Derating
- 8 Application and Implementation
- 9 Layout
- 10器件和文檔支持
7.3.9 Soft Start
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. Upon turnon, after VCC reaches the undervoltage threshold, an 8-µA internal current source charges up an external capacitor CSS connecting to the SS pin. The ramping voltage at the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage VOUT in a controlled manner.
The soft start time duration to reach steady state operation is given by the formula:
This equation can be rearranged as follows:
Use of a 4.7-nF capacitor results in a 0.5-ms soft-start duration. This is a recommended value. Note that high values of CSS capacitance causes more output voltage drop when a load transient goes across the DCM-CCM boundary. If a fast load transient response is desired for steps between DCM and CCM mode the softstart capacitor value must be less than 18 nF (which corresponds to a soft-start time of 1.8 ms).
An internal switch grounds the SS pin if any of the following three cases happens: (i) VCC is below the UVLO threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in Figure 19.
Figure 19. Alternate Shutdown Implementation