L6995 Ver la hoja de datos (PDF) - STMicroelectronics
Número de pieza
componentes Descripción
Lista de partido
L6995 Datasheet PDF : 25 Pages
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L6995
Eq 6
C-----I--N----T---2- = ∆----V----O----U----T--
CINT1
VINT
Where ∆VOUT is the output ripple and ∆VINT is the ripple wanted at the INT pin (100mV typ).
Figure 4. Integrator loop block diagram
Vin
R1
R2
Cint2
One-shot generator
OSC
FFSR
RQ
From Vsense
Vref
SQ
HGATE
FB
+
-
LGATE
PWM comparator
HS
LS DS
Vref
-
INT
+
-
+
Integrator amplifier
Vsense
Gndsense
Cint1
PCB TRACES
Vout
LOAD
Respect to a traditional PWM controller, that has an internal oscillator setting the switching frequency, in a hys-
teretic system the frequency can change with some parameters (input voltage, output current). In L6995 is im-
plemented the voltage feed-forward circuit that allows constant switching frequency during steady-sate
operation with the input voltage variation. There are many factors affecting switching frequency accuracy in
steady-state operation. Some of these are internal as dead times, which depend on high side MOSFET driver.
Others related to the external components as high side MOSFET gate charge and gate resistance, voltage
drops on supply and ground rails, low side and high side RDSON and inductor parasitic resistance.
During a positive load transient, (the output current increases), the converter switches at its maximum frequency
(the period is TON+TOFFmin) to recover the output voltage drop. During a negative load transient, (the output
current decreases), the device stops to switch (high side MOSFET remains off).
1.3 Transition from PWM to PFM/PSK
To achieve high efficiency at light load conditions, PFM mode is provided. The PFM mode differs from the PWM
mode essentially for the off section; the on section is the same. In PFM after a turn-on cycle the system turns-
on the low side MOSFET, until the inductor current reaches the zero A value, when the zero-crossing compar-
ator turns off the low side MOSFET. In this way the energy stored in the output capacitor will not flow to ground,
through the low side MOSFET, but it will flow to the load. In PWM mode, after a turn on cycle, the system keeps
the low side MOSFET on until the next turn-on cycle, so the energy stored in the output capacitor will flow
through the low side MOSFET to ground. The PFM mode is naturally implemented in hysteretic controller, in
fact in PFM mode the system reads the output voltage with a comparator and then turns on the high side MOS-
FET when the output voltage goes down a reference value. The device works in discontinuous mode at light
load and in continuous mode at high load. The transition from PFM to PWM occurs when load current is around
half the inductor current ripple. This threshold value depends on VIN, L, and VOUT. Note that the higher the in-
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