米6体育平台手机版

ZHCSFB5D July 2016 – December 2017 LM5141-Q1

PRODUCTION DATA.

封装选项

请参考 PDF 数据表获取器件具体的封装图。

机械数据 (封装 | 引脚)

RGE|24

散热焊盘机械数据 (封装 | 引脚)

订购信息

7.3.3 Oscillator

The LM5141-Q1 has an internal trimmed oscillator with two frequency options: 2.2 MHz, or 440 kHz. With the OSC pin connected to VDDA the oscillator frequency is 2.2 MHz. With the OSC pin grounded, the oscillator frequency is 440 kHz. The state of the OSC pin is read and latched during VCC power-up and cannot be changed until VCC drops below the VCC_(UVLO) threshold.

The oscillator frequency can be modulated up or down from the nominal oscillator frequency (2.2 MHz or 440 kHz) on demand by connecting a resistor from the RT pin to ground (refer to Figure 19). To disable the frequency modulation option, the RT pin can be grounded or left open. If the RT pin is connected to ground during power-up the frequency modulation option is latch-off and cannot be changed unless VCC is allowed to drop below the VCC_(UVLO) threshold. If the RT pin is left open during power-up the frequency modulation option will be disabled, but it can be enabled at a later time by switching in a valid RT resistor. When the frequency modulation option is disabled, the LM5141-Q1 will operate at the internal oscillator frequency (2.2 MHz or 440 kHz).

On power up, after soft-start is complete and the output voltage is in regulation, a 16-µs timer is initiated. If a valid RT resistor is connected, the LM5141-Q1 will switch to the frequency set by the RT resistor n the completion of the 16-µs time delay.

The modulation range for 2.2 MHz is 1.8 MHz to 2.53 MHz (refer to Table 1). If an RT resistor value > 95 kΩ (typical) is placed on the RT pin, the LM5141-Q1 controller will assume that the RT pin is open, and will use the internal oscillator. If an RT resistor < 27 kΩ (typical) is connected, the controller will use the internal oscillator. To calculate an RT resistor for a specific oscillator frequency, use Equation 1 for the 2.2 MHz frequency range or Equation 2 for the 440 kHz frequency range.

Equation 1. LM5141-Q1 equation_01_snvsaj6.gif

where

RT is kΩ and Fsw is in MHz

Equation 2. LM5141-Q1 equation_02_snvsaj6.gif

where

RT is in kΩ and Fsw is in kHz

Table 1. RT Resistance vs Oscillator Frequency

S1	S2	RT RESISTANCE (TYPICAL) 2.2 MHz	2.2 MHZ OSCILLATOR RANGE (TYPICAL)	RT RESISTANCE (TYPICAL) 440 kHz	440 kHz OSCILLATOR RANGE (TYPICAL)
X	X	> 95 kΩ	internal oscillator	> 95 kΩ	internal oscillator
OFF	OFF	61.98 kΩ total	1.8 MHz	73.8 kΩ total	300 kHz
OFF	ON	50.18 kΩ total	2.2 MHz	50.1 kΩ total	440 kHz
ON	OFF	43.2 kΩ	2.53 MHz	44.2 kΩ	500 kHz
X	X	< 27 kΩ	internal oscillator	< 27 kΩ	internal oscillator

LM5141-Q1 rt_connection_circuit_2pt2mhz_snvsaj6.gif

Figure 19. RT Connection Circuit, 2.2 MHz

LM5141-Q1 rt_connection_circuit_440mhz_snvsaj6.gif

Figure 20. RT Connection Circuit, 440 kHz

An alternative method to modulate the oscillator frequency is to use an analog voltage connected to the RT pin through a resistor. See Figure 21. An analog voltage of 0.0 V to 0.6 V modulates the oscillator frequency between 1.8 MHz to 2.53 MHz (OSC at 2.2 MHz), or 300 kHz to 500 kHz (OSC at 440 kHz). The analog voltage source must be able to sink current.

LM5141-Q1 analog_voltage_control_oscillator_frequency_snvsaj6.gif

Figure 21. Analog Voltage Control of the Oscillator Frequency

When the LM5141-Q1 is in the low I_Q standby mode, the controller sets the RT pin to a high impedance state and ignores the RT resistor. After coming out of standby mode, the controller will monitor the RT pin. If a valid resistor is connected, and there have been 16 µs of continuous switching without a zero-crossing event, the LM5141-Q1 will switch to the frequency set by the RT resistor.