LTC3899 operates at half switching frequency

Hi Team,

LTC3899 SW2 output is not operating at expected switching frequency (550kHz) over the input voltage range (9-36). At low input voltage (12 V) switching frequency is as expected but at higher input voltage (28 V) every second switching cycle is missing. During "faulty" operation error amplifier output (ITH2) seems to go to low value at end of SW2 switching cycle. Below you can find few measurement results. SW1 is measured as a reference purposes only.

Measurement 1 (input voltage 12 V) - circuit operates as expected

Measurement 2 (input voltage 28 V) - every second SW2 switching cycle is skipped

We are using LTC3899 in following circuitry (attached). Input voltage is 9-36 V and output voltages are:
SW1 (buck): 5.1 V
SW2 (buck): 3.3 V
SW3 (boost): 12.1 V (input from SW1 output)

Controller is set to forced continuous mode (MODE tied to INTVCC). SW2 (+3.3V) output current during test is 95 mA.

PDF

What may cause the SW2 to skip pulses although set to forced continuous mode?

BR,

 Tero