Part Number:LM2678
Hi,
I am designing a power stage using LM2678 where:
Vin: 19V
Vout_min: 2V
Vout_max: 16V
Iout: 5A.
Problem: With a 4ohm load I found that it can not go higher than 7V approx (1.75A output). With a 8 ohm load this limit appears at 10V approx. With lighter loads (16ohm ) works fine.
Half-Solution: I found Samuel Jaffe recommended increasing input capacitance as closer as possible to the Switcher (Input and GND pins). (e2e.ti.com/.../699199)
I tried adding 1uF ceramic and 10uF ceramic caps. First soldered on the pins and later on a position where would be more convinient for manufacturing. All cases helped in the same way.
New limit is on the 10V (with a 4ohm load = 2.5A output). Moreover if power stage is turned-on configured (Rfeedback) for a higher voltage will do it right. The problem is rising voltage while it is running (changing Rfeedback). Then looses tracking and falls back to 10V.
Detail: I am using an SPI configurable resistor. C59 is on the way Vin goes to the switcher, so is filters the feeded19V to both ICs.
Find schema, clean layout, added caps layout attached.
Any recommendation/advice will be really apreciated.
Thanks in advance.
Tom