L6740L Datasheet, PDF (36/44 Pages) STMicroelectronics – Hybrid controller (4+1) for AMD SVID and PVID processors

Main oscillator
8
Main oscillator
L6740L
The controller embeds a dual-Oscillator: one section is used for the CORE and it is a
multiphase programmable oscillator managing equal phase-shift among all phases and the
other section is used for the NB section. Phase-Shift between the CORE and NB ramps is
automatically adjusted according to the CORE phase # programmed.
The internal oscillator generates the triangular waveform for the PWM charging and
discharging with a constant current an internal capacitor. The switching frequency for each
channel, F_SW, is internally fixed at 150kHz: the resulting switching frequency for the CORE
section at the load side results in being multiplied by N (number of configured phases).
The current delivered to the oscillator is typically 22µA (corresponding to the free running
frequency F_SW=150kHz) and it may be varied using an external resistor (R_OSC) typically
connected between the OSC pin and SGND. Since the OSC pin is fixed at 1.240V, the
frequency is varied proportionally to the current sunk from the pin considering the internal
gain of 6.8KHz/µA (See Figure 15).
Connecting R_OSC to SGND the frequency is increased (current is sunk from the pin),
according to the following relationships:
F_SW
=
150
kHz
+
-----¹---^.--²---⁴----⁰---^V-------
R_OSC(kΩ)
⋅
6.8
-^k-_µ--^H--_A---^z-
=
150kHz + -⁸---^.-⁴----³---²-----^â‹---¹---⁰----⁶-
R_OSC(kΩ)
Figure 15. R_OSC vs. switching frequency
1000
900
800
700
600
500
400
300
200
100
10
100
Rosc Resistor [kOhms]
1000
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