LM5145 datasheet(24/69 Pages) TI

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If a low IQ mode is required, take care when selecting the external resistors. The extra current drawn from the

external divider is added to the LM25143 ISTANDBY current (15 µA typical). The divider current reflected to VIN is

divided down by the ratio of VOUT/VIN. For example, if VOUT is set to 5.55 V with RFB1 equal to 82.5 kΩ and RFB2

equal to 10 kΩ, use Equation 7 to calculate the input current from a 12-V input required to supply the current in

the feedback resistors.

OUT

VIN(DIVIDER)

FB1

FB2

VIN

STANDBY

VIN(DIVIDER)

5.55 V

35 $

82.5k

10k

80% 12 V

15 $

(7)

If one output is enabled and the other disabled, the VCC output is in regulation. The HB voltage of the disabled

channel charges to VCC through the bootstrap diode. As a result, the HO driver bias current (approximately 1.5

µA) can increase the output voltage of the disabled channel to approximately 2.2 V. If this is not desired, add a

load resistor (100 kΩ) to the output that is disabled to maintain a low-voltage OFF state.

9.3.11 Minimum Controllable On Time

There are two limitations to the minimum output voltage adjustment range: the LM25143 voltage reference of 0.6

V and the minimum controllable switch-node pulse width, tON(min).

tON(min) effectively limits the voltage step-down conversion ratio of VOUT/VIN at a given switching frequency. For

fixed-frequency PWM operation, the voltage conversion ratio must satisfy Equation 8.

OUT

ON(min)

(8)

where

• tON(min) is 65 ns (typical).

• FSW is the switching frequency.

If the desired voltage conversion ratio does not meet the above condition, the LM25143 transitions from fixed

switching frequency operation to a pulse-skipping mode to maintain output voltage regulation. For example, if

the desired output voltage is 5 V with an input voltage is 24 V and switching frequency of 2.1 MHz, the voltage

conversion ratio test in Equation 9 is satisfied.

5 V

65 ns 2.1 MHz

24 V

0.208

0.137

(9)

For wide VIN applications and low output voltages, an alternative is to reduce the LM25143 switching frequency

to meet the requirement of Equation 8.

9.3.12 Error Amplifier and PWM Comparator (FB1, FB2, COMP1, COMP2)

Each channel of the LM25143 has an independent high-gain transconductance amplifier that generates an

error current proportional to the difference between the feedback voltage and an internal precision reference

(0.6 V). The output of the transconductance amplifier is connected to the COMP pin, allowing the user to

provide external control loop compensation. A type-II compensation network is generally recommended for peak

current-mode control.

The amplifier has two gain settings, one is for normal operation with a gm of 1200 µS and the other is for

ultra-low IQ with a gm of 60 µS. For normal operation, connect MODE to AGND. For ultra-low operation IQ,

connect MODE to AGND through a 10-kΩ resistor.

LM25143

SNVSC10 – MARCH 2022

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LM5145 Datasheet(PDF) 24 Page - Texas Instruments

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