L5965 datasheet(31/73 Pages) STMICROELECTRONICS

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4.2.16

Maximum Duty Cycle and Refresh Mode for Buck

The high-side N-channel MOSFET is turned on at the beginning of each clock cycle and kept on until the inductor

current reaches its peak value as set by the regulation loop. Once the high side power is turned OFF, and after a

small delay (shoot-through delay), the lower N-channel MOSFET is turned on until the start of the next clock

cycle.

In dropout operation the high-side MOSFET can stay on 100%. To ensure the bootstrap capacitor is recharged,

the buck low-side power is forced off time (140ns) per 4 cycles. It is called “refresh mode”. This forced OFF time

limits the maximum duty cycle of the buck to:

Dmax=1−fsw⋅tOFF⋅0.25

(5)

The actual maximum duty cycle varies with the switching frequency.

Figure 19. Refresh mode in bucks

4 cycles

fSW_buck

PH_buck

tOFF_refresh

GADG0509181611PS

4.2.17

Frequency-Hopping Spread Spectrum

L5965 features a pseudo-random spectrum for 2.4 MHz switching frequency, and a triangular spread architecture

for 400 kHz switching frequency. The frequency shifts only by one step at each cycle to avoid large jumps in

bucks and boost switching frequencies.

L5965

Voltage regulators and features description

DS12567 - Rev 3

page 31/73

L5965 Datenblatt(PDF) 31 Page - STMicroelectronics