L6995
8/25
Eq 6
Where
∆
V
OUT
is the output ripple and
∆
V
INT
is the ripple wanted at the INT pin (100mV typ).
Figure 4. Integrator l oop block diagram
Respect to a traditional PWM controller, that has an internal osci llator setting the sw itchi ng 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 RDS ON and inductor parasitic resistance.
During a posi tive load trans ient, (the output current incr eases), the converter switches at its maxi mum 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 PW M t o PFM /PSK
To achieve high efficienc y at l ight 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 M OSFET. In this w ay the energy stored in the output capacitor wi ll not flow to gr ound,
through the l ow side MO SFET, but it w ill flow to th e load. In P WM mode, after a tur n on cy cle, the sy stem k eeps
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 P FM mode the sys tem reads the output vol tage 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 i n continuous mode at high l oad. The transi tion fr om PFM to PWM oc curs w hen lo ad current is around
half the inductor current ripple. This threshold value depends on V
IN
, L, and V
OUT
. Note that the higher the in-
C
INT2
C
INT1
--------------- -
V
OUT
∆
V
INT
------------------=
PCB TRACES
LOAD
F rom Vsense
Q
Cint1
R1
INT
Vref
FB
DS
R
S
V out
Vin
HGATE
LGATE
Q
OSC
Vref
One-shot generator
FFSR
PWM comparator
Vsense
Gndsense
R2
Cint2
HS
LS
Integrator amplifier
+
-
-
+
+
-
