LM5085SDESLASHNOPB datasheet(18/33 Pages) TI

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RADJ =

32 PA

6.5A x 0.01:

= 2.03 k:

IPK

IOUT

IOR

1/FS

L1 =

tON(min) x (VIN(max) - VOUT)

IOR(max)

= 14.9 PH

RT =

5 x (12 - 1.56V)

1.45 x 10

-7

x 12 x 300 kHz

- 1.4 = 90.9

(50 ns + 57 ns) x (12 - 1.56V)

1.45 x 10

-7

LM5085, LM5085-Q1

SNVS565I – NOVEMBER 2008 – REVISED AUGUST 2015

www.ti.com

Typical Application (continued)

For this example, RFB2/RFB1 = 3. Typically, RFB1 and RFB2 should be chosen from standard value resistors in the

range of 1k

Ω to 20kΩ which satisfy the above ratio. For this example, RFB2 = 10kΩ, and RFB1 = 3.4kΩ.

RT, PFET: Before selecting the RT resistor, the PFET must be selected as its turn-on and turn-off delays affect

the calculated value of RT. For the Vishay Si7465 PFET, the difference of its typical turn-off and turn-on delays is

57ns. Using Equation 5 at nominal input voltage, RT calculates to be:

(16)

A standard value 90.9k

Ω resistor is selected. Using Equation 3, the minimum on-time at the PGATE pin, which

occurs at maximum input voltage (55V), is calculated to be 300ns. This minimum one-shot period is sufficiently

longer than the minimum recommended value of 150ns. The minimum on-time at the SW node (junction of Q1,

D1, L1) is longer due to the delay added by the PFET (57ns). Therefore the minimum SW node on-time is 357ns

at 55V. The maximum on-time at the SW node is calculated to be 2.55µs at 7V.

L1: The main parameter controlled by the inductor value is the current ripple amplitude (IOR). See Figure 26. The

minimum load current for continuous conduction mode is used to determine the maximum allowable ripple such

that the inductor current valley does not fall to zero. Continuous conduction mode operation at minimum load

current is not a requirement of the LM5085, but serves as a guideline for selecting L1. For this example, the

maximum ripple current is:

IOR(max) = 2 x IOUT(min) = 1.2 Amp

(17)

If the minimum load current of the application is zero, a good initial estimate for the maximum ripple current

(IOR(max)) is 20% of the maximum load current. The ripple calculated in Equation 17 is then used in the following

equation to calculate L1:

(18)

A standard value 15µH inductor is selected. Using this inductance value, the maximum ripple current amplitude,

which occurs at maximum input voltage, is calculated to be 1.19 Ap-p. The peak current (IPK) at maximum load

current is 5.6A. However, the current rating of the selected inductor must be based on the maximum current limit

value calculated below.

Figure 26. Inductor Current Waveform

RSEN, RADJ: To achieve good current limit accuracy and avoid over designing the power stage components, the

sense resistor method is used for current limiting in this example. A standard value 10m

Ω resistor is selected for

RSEN, resulting in a 50mV drop at maximum load current, and a maximum 0.25W power dissipation in the

resistor. Since the LM5085 uses peak current detection, the minimum value for the current limit threshold must

be equal to the maximum load current (5A) plus half the maximum ripple amplitude calculated above:

ICL(min) = 5A + 1.19A/2 = 5.6A

(19)

At this current level the voltage across RSEN is 56mV. Adding the current limit comparator offset of 9mV (max)

increases the required current limit threshold to 6.5A. Using Equation 9 with the minimum value for the ADJ pin

current (32µA), the required RADJ resistor is calculated to be:

(20)

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