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L6995 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 6 Page - STMicroelectronics

๋ถ€ํ’ˆ๋ช… L6995
์ƒ์„ธ๋‚ด์šฉ  STEP DOWN CONTROLLER FOR HIGH DIFFERENTIAL INPUT-OUTPUT CONVERSION
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์ œ์กฐ์‚ฌ  STMICROELECTRONICS [STMicroelectronics]
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L6995 ๋ฐ์ดํ„ฐ์‹œํŠธ(HTML) 6 Page - STMicroelectronics

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1
DEVICE DESCRIPTION
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 KOSC = 250ns and ฯ„ is the internal propagation delay time (typ. 70ns). 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 the OSC pin to a voltage partition from VIN to GND, it allows a
steady-state switching frequency FSW independent of VIN. It results:
Eq 2
where
Eq 3
Eq 4
The above equations allow setting the frequency divider ratio
ฮฑOSC once output voltage has been set; note that
such equations hold only if VOSC<2V. Further the Eq2 shows how the system has a switching frequency ideally
independent from the input voltage. The delay introduces a light dependence from VIN. A minimum off-time con-
strain of about 580ns is introduced in order to assure the boot capacitor charge and to limit the switching fre-
Q
Vsense
R3
R4
R2
R1
R
S
Vout
Vin
HGATE
LGATE
Q
OSC
FB
Vref
HS
LS
DS
PWM comparator
FFSR
One-shot generator
-
+
T
ON
K
OSC
V
SENSE
V
OSC
----------------------
ฯ„
+
=
f
SW
V
OU T
V
IN
---------------
1
T
ON
-----------
ฮฑ
OS C
ฮฑ
OUT
--------------
1
K
OSC
---------------
ฮฑ
OSC
โ†’
f
SWKOSC ฮฑOUT
==
=
ฮฑ
OSC
V
OSC
V
IN
---------------
R
2
R
2
R
1
+
--------------------
==
ฮฑ
OUT
V
FB
V
OUT
---------------
R
4
R
3
R
4
+
--------------------
==


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