ZHCSFE3B March 2016 – November 2017 LM5161
PRODUCTION DATA.
As mentioned earlier in Soft-Start, the SS capacitor CSS, must be more than 1 nF in both Buck and Fly-Buck applications. Apart from determining the start-up time, this capacitor serves for the external compensation of the internal GM error amplifier. A minimum value of 1 nF is necessary to maintain stability. The SS pin must not be left floating.
When the FPWM pin is shorted to ground or left unconnected, no external ripple injection is necessary in a Buck application. Should an external feedback ripple circuit be configured when FPWM = 0, it will produce higher ripple at the output.
Add a resistor (>3Ω) in series with the BST capacitor when using the part in FPWM = 0, as described in detail in Forced Pulse Width Modulation (FPWM) Mode.
When configured as a Fly-Buck, the FPWM pin must always be connected to VCC. A Fly-Buck application must operate in the continuous conduction mode all the time in order to maintain adequate voltage regulation on the secondary side. FPWM = 0 is not a valid mode in the Fly-Buck application.