L6714 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
L6714
STMicroelectronics
'L6714' PDF : 70 Pages View PDF
Oscillator
19 Oscillator
L6714
L6714 embeds four phase oscillator with optimized phase-shift (90º phase-shift) in order to
reduce the input rms current and optimize the output filter definition.
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, FSW, is internally fixed at 150kHz so that the resulting switching frequency at the
load side results in being multiplied by N (number of phases).
The current delivered to the oscillator is typically 25µA (corresponding to the free running
frequency FSW = 150kHz) and it may be varied using an external resistor (ROSC) connected
between the OSC pin and SGND or VCC (or a fixed voltage greater than 1.24V). Since the
OSC pin is fixed at 1.24V, the frequency is varied proportionally to the current sunk (forced)
from (into) the pin considering the internal gain of 6KHz/µA.
In particular connecting ROSC to SGND the frequency is increased (current is sunk from the
pin), while connecting ROSC to VCC = 12V the frequency is reduced (current is forced into
the pin), according the following relationships:
ROSC vs. SGND
FSW
=
150 ( k H z )
+
-----1---.--2---4---0----V-------
R O S C ( k Ω)
⋅
6 k--µ--H--A---z-
=
150 ( k H z )
+
-7---.-4----2---2-----⋅------1---0----3-
R O S C ( k Ω)
⇒R· OSC(kΩ)
=
-----------
FSW
--H-----z-
µA
=
150 ( k H z )
+
-7---.-4----2---2-----⋅------1---0----3-
R O S C ( k Ω)
⇒R· OSC(kΩ)
=
---------------7---.--4---2---2-----⋅------1---0----3--------------- [kΩ]
FSW(kHz) – 150(kHz)
ROSC vs. +12V
FSW = 150(kHz) – 1----2-R---V-O----–S----C-1---(.--2k---4Ω---0--)--V-- ⋅
6 k--µ--H--A---z-
=
150 ( k H z ) – -6---.-4----5---6-----⋅------1---0----4-
R O S C ( k Ω)
⇒RO S C ( k Ω)
=
-----
15
6 k--µ--H--A---z-
=
150 ( k H z ) – 6----.-4----5---6-----⋅------1---0----4-
R O S C ( k Ω)
⇒R O S C ( k Ω)
=
---------------6---.--4---5---6-----⋅------1---0----4--------------- [kΩ]
150(kHz) – FSW(kHz)
When using the Low-Side MOSFETs current sense, the maximum programmable switching
frequency per phase must be limited to 500kHz to avoid current reading errors causing, as a
consequence, current sharing errors.
Anyway, device power dissipation must be checked prior to design high switching frequency
systems.
54/70
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