l6995 STMicroelectronics, l6995 Datasheet - Page 6

• Manufacturer Part Number: l6995
• Description: Step Down Controller For High Differential Input-output Conversion
• Manufacturer: STMicroelectronics

L6995
1
1.1 Constant On Time PWM topology
Figure 2. Loop block schematic diagram
The device implements a Constant On Time control scheme, where the Ton is the high side MOSFET on time
duration forced by the one-shot generator. The on time is directly proportional to VSENSE pin voltage and in-
verse to OSC pin voltage as in Eq1:
Eq 1
where K
state a minimum on time corresponding to V
responding Ton will not decrease. Connecting the OSC pin to a voltage partition from V
steady-state switching frequency F
Eq 2
where
Eq 3
Eq 4
The above equations allow setting the frequency divider ratio
such equations hold only if V
independent from the input voltage. The delay introduces a light dependence from V
strain of about 580ns is introduced in order to assure the boot capacitor charge and to limit the switching fre-
6/25
DEVICE DESCRIPTION
OSC
Vin
T
f
O SC
OUT
SW
O N
R2
= 250ns and
R1
=
=
=
=
V
-------------- -
K
-------------- -
V
V
V
-------------- -
OUT
V
O SC
V
Vsense
OUT
IN
O SC
FB
IN
OSC
R4
---------- -
T
V
--------------------- - +
1
O N
V
=
SENSE
=
One-shot generator
O SC
OSC
is the internal propagation delay time (typ. 70ns). The system imposes in steady
--------------------
R
--------------------
R
Vref
=
3
2
R
R
<2V. Further the Eq2 shows how the system has a switching frequency ideally
+
FB
+
--------------
+
-
4
2
OS C
OUT
SW
R
R
R3
PWM comparator
4
1
independent of V
-------------- -
K
S
O SC
R
FFSR
1
OSC
Q
Q
= 2V. In fact if the V
O SC
HGATE
LGATE
=
IN
. It results:
f
S W
HS
LS
K
OSC
O SC OUT
once output voltage has been set; note that
DS
OSC
voltage increases above 2V the cor-
IN
. A minimum off-time con-
IN
to GND, it allows a
Vout