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LM3429 has very high CSH voltage

Other Parts Discussed in Thread: LM3429

I am designing a tunable laser driver. My plan was to use the LM3429 as it accommodates high input voltage and has analog dimming (I can’t use PWM dimming in this application). The basic design parameters are as follows:

Vin = 24V

Vout = 4.1 V

ILED = 0.43A

 

Using the Buck configuration circuit on page 42, I created a calculator to determine the value of the support components.

I’ve put together a prototype, but it is not switching. The nDIM pin is at 2.0V, and the OVP pin is at 0V. What I’ve noticed is that the CSH pin is at 4.72V. Rsch = 12.4k as in the datasheet. I’ve calculated the Vsns = 0.282 and Rhsp = Rhsn = 2.8k for ILED = 0.43A. If I’m reading the data sheet correctly, the voltage at the CSH pin should be 1.24V. Why is it 4.72V?

As an added bit of info, I can lower the voltage at CSH by lowering the value of Rcsh.

Here are the component values:

 

Rt = 47k

Q1 = FDT1600N10

Ct = 1000pF

L1 = 100uH

Ccmp = 10uF

D1 = SSA

Rcsh = 12.4k

Rlim = 0.15

Rhsp = Rhsn = 2.82k

Cbyp = 2.2uF

Rsns = 0.66

 

Rfs = 10

 

Cfs = 2700pF