LM5145 datasheet(21/73 Pages) TI2

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A 12Ω internal resistor connects VDDA to VCC. Bypass VDDA to AGND with a 1µF ceramic capacitor to achieve

a low-noise internal bias rail.

If the BIAS1/VOUT1 voltage is above 4.3V, BIAS1/VOUT1 internally connects to a second input of the VCC

regulator. This action helps to reduce the internal power dissipation of the LM25137, as bias current derives from

VOUT1 instead of VIN. Avoid connecting BIAS1/VOUT1 or VOUT2 to a voltage greater than 36V or less than

–0.3V.

7.3.3 Precision Enable (EN1, EN2)

The LM25137 has a precision enable circuit on each EN input. When the EN1 or EN2 voltage is greater than

1V, switching is enabled on the respective channel. If EN1 and EN2 are pulled below 0.5V, the LM25137 is in

shutdown with an IQ of 3.6μA (typical) current consumption from VIN. When the EN1 or EN2 voltage is between

0.6V and 1V, the LM25137 is in standby mode with the VCC regulator active but the controller is not switching.

The input quiescent current in standby mode is 260μA typical.

Enable the LM25137 with a EN1/2 voltage greater than 1V. However, many applications benefit from using a

resistor divider RUV1 and RUV2, as shown in Figure 7-2, to establish a precision UVLO level. Add RUV3 in series

to provide additional voltage hysteresis. Avoid leaving the EN1 and EN2 pins floating. Keep the EN1/2 voltage

under 42V.

Use Equation 1 to calculate the UVLO resistors, where VIN(on) and VIN(off) are the required input voltage turn-on

and turn-off thresholds.

(1)

EN1/2

RUV2

RUV1

Precision enable

comparator

10µA

VDDA

VIN

0.95V

RUV3

Disable signal

from MCU

Figure 7-2. Programmable Input Voltage UVLO Turn-On, Turn-Off

7.3.4 Switching Frequency (RT)

Program the LM25137 oscillator with a resistor from RT to AGND to set the free-running switching frequency

between 100kHz and 2.2MHz. Use Equation 2 to calculate the RT resistance for a given switching frequency.

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