L6996 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6996' PDF : 26 Pages View PDF
L6996
1 DEVICE DESCRIPTION
1.1 Constant On Time PWM Topology
Figure 12. Loop block schematic diagram
Vin
R1
One-shot generator
OSC
R2
CS-
FFSR
RQ
HS
HGATE
SQ
VID0-4
DAC
LS
DS
LGATE
Vprog
VFB+
VFB-
+
--
PWM comparator
Rsense
Vout
This device implements a Constant On Time control, where the Ton is the on time duration forced by a one-shot
circuit. The controller calculates the one-shot time directly proportional to the VCS- pin voltage and inversely to
the OSC pin voltage as in Eq 1:
Eq 1
TON
=
KO
S
C
--V----C----S-----
VOSC
+
τ
where KOSC=180ns and τ is the internal propagation delay time (Typ. 40ns). The system imposes in steady
state a minimum on time corresponding to VOSC = 2V. In fact if the VOSC voltage increases above 2V the cor-
responding Ton will not decrease. Connecting OSC pin to a voltage partition from VIN to GND, it allows steady-
state switching frequency FSW independent of VIN. It results:
Eq 2
FSW = V---V--O--I--UN----T- ⋅ T----O-1----N- → αOSC = FSW ⋅ KOSC
where
Eq 3
αOSC
=
V-----O----S----C-
VIN
=
-------R-----2--------
R2 + R1
The above equations allow setting the frequency divider ratio aOSC once output voltage has been set; note that
such equations hold only if VOSC<2.A minimum off-time constrain of about 500nS is introduced in order to as-
sure the boot capacitor charge and to limit switching frequency after a load transient as well as to mask PWM
comparator output against switching noise and spikes.
The system has not an internal clock, because this is a hysteretic controller, so the turn on pulse will start if three
10/26
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