S

ILIM

P

0.25 V

0.25 V

R11

3.2

N

I

1

1.24

1

1

I

15

2

N

CSR

2

2

100

=

=

=

W

D

æ

ö

æ

ö

+

´

´

+

´

´

ç

÷

ç

÷

è

ø

è

ø

HICCUP

SS

t1

1.11

D

1.09 %

t1

t2

t

1.11 100

1

=

=

=

+

+

+

+

1.0 V

C12

1.0 V

100 nF

t2

100 ms

1 A

1 A

æ

ö æ

ö

´

´ ´

=

=

=

ç

÷ ç

÷

m

m

è

ø è

ø

2.0 V

C13

2.0 V

10 nF

t1

1.11 ms

18

A

18

A

æ

ö æ

ö

´

´

=

=

=

ç

÷ ç

÷

m

m

è

ø è

ø

( )

SS dly

1.0 V C12

1.0 V 0.1 F

t

1.0 ms

100 A

100 A

æ

ö æ

ö

´

´

m

=

=

=

ç

÷ ç

÷

m

m

è

ø è

ø

LM5037

SNVS578D – NOVEMBER 2008 – REVISED MAY 2015

www.ti.com

Use the nearest E96 value of 140 k

Ω.

8.2.2.7 Soft-Start Delay

Use Equation 21 to calculate the time period from soft-start delay to commencement of first PWM switching.

(21)

After the soft-start delay period, the soft-start ramp time depends on the power stage design and the operating

conditions (input voltage and output load).

8.2.2.8 Overload Timer

With a timing capacitance of 10 nF on the RES pin (C13), calculate hiccup-mode timing and duty cycle for a

sustained over-current condition using Equation 22 and Equation 23. Time period t1 describes the hiccup-mode

current-limit persist time and time period t2 describes hiccup-mode cool-down off-time.

(22)

(23)

Calculate the hiccup-mode duty cycle using Equation 24.

(24)

8.2.2.9 Current Sense

In order to improve the efficiency, a current sense transformer (T2) is used. This transformer uses a 1:100 step-

down ratio. This ratio reduces the power dissipation in the current-sense resistor, R11.

Set the current-limit point after calculating the

•

output inductor peak-to-peak ripple current when the device operates in CCM

•

•

the turns ratios of the current-sense transformer (CSR)

Using the full load output current of 10 A, and the current limit target of 150%, or 15 A, calculate the required

value for R11. The R11 resistance must generate a voltage at the CS pin to equal the internal cycle-by-cycle limit

of nominally 0.25 V at the current limit level at the output.

(25)

Round the calculated R11 resistance to a standard 3

Ω. If the power stage operates heavily in CCM, the output

inductor ripple component can be ignored to yield a good first-order approximation to the required value.

8.2.2.10 Output Voltage Feedback

Because the output of the DC-DC converter is isolated from the input, this design uses a secondary-side

reference (U5) and error amplifier (U3). The output of the error amplifier, U3 represents the required demand

level to maintain regulation as a function of output load and input line. The design couples the demand signal

across the isolation barrier to the primary through opto-coupler, U6.

Because the error amplifier and reference reside on the secondary side, this design disregards the internal

reference and error amplifier features of the LM5037 device. The FB pin of the LM5037 device is connected to

GND, which forces the COMP pin to pull up to approximately 5 V through the internal 5-k

Ω pull-up resistance.

The opto-coupler, U6 then externally pulls down on the COMP pin to set the required level to achieve the

required duty cycle at any given load or line level.

30

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