L5994A STMicroelectronics, L5994A Datasheet - Page 16

• Manufacturer Part Number: L5994A
• Description: IC CTRLR PS ADJ TRPL-OUT 32-TQFP
• Manufacturer: STMicroelectronics

Specifications of L5994A

• Pwm Type: Current Mode
• Number Of Outputs: 3
• Frequency - Max: 345kHz
• Duty Cycle: 96%
• Voltage - Supply: 4.75 V ~ 25 V
• Buck: Yes
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 140°C
• Package / Case: 32-TQFP, 32-VQFP
• Frequency-max: 345kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-5340; L5994A

L5994 - L5994A
Power MOSFET's and Schottky Diodes
Since the gate drivers of the device are powered by a 5V bus , the use of logic-level MOSFET's is highly rec-
ommended, especially for high current applications. Their breakdown voltage V
V
The R
power MOSFET's as the main switch and the synchronous rectifier, the total power they dissipate does not de-
pend on the duty cycle. Thus, if PON is this power loss (few percent of the rated output power), the required
RDS(ON) (@ 25 °C) can be derived from:
where I
of R
worth noticing, however, that generally the lower R
higher gate drive consumption. In fact, each switching cycle, a charge Q
ground, resulting in an equivalent drive current:
which affects efficiency at low load currents. Besides, this current is drawn from the PREG5 line whose source
capability, I
charge (@V
assuming four identical MOSFET's.
The Schottky diode to be placed in parallel to the synchronous rectifier must have a reverse voltage V
er than V
than Iout. The selection criterion should be:
Sense Resistors
The sense resistor of each section is selected according to their respective maximum output current. The cur-
rent sense comparator limits the inductor peak current and therefore the maximum DC output current is the peak
value less half of the peak-to-peak ripple. The intervention threshold is set at 50mV for both sections, thus the
resistor values should be:
Since the comparator threshold that triggers pulse-skipping mode is 11mV, the output current at which the sys-
tem enters this kind of operation is approximately one fourth of the maximum output current. The sense resistors
values are in the low milliohms thus it is important to take correctly the current sense signals. Make sure that
the Kelvin connections between the current sense pins of the IC and the sense resistor do not carry the output
current.
16/26
INMAX
DS(ON)
DS(ON
out
with a certain margin, so the selection will address 20V or 30V devices.
INMAX
is either I
(typically, a = 5 · 10
SRC
) can be selected once the allowable power dissipation has been established. By selecting identical
GS
. Since it conducts for less than 5% of the switching period, the current rating can be much lower
= 5V):
(25mA min), must not be exceeded, thus a further constraint concernes the MOSFET total gate
TOT5
or I
R
SENSE5
OUT3
-3
, according to the section under consideration, a is the temperature coefficient
°C
-1
=
for these low-voltage classes) and T the admitted temperature rise. It is
V
R
t(Schottky)
-------------- m
I
L5 PK
DS ON
50
Q
< V
I
=
g
g
= Q
DS(ON)
---------------------------------------------- -
I
t(body-diode)
2
OUT
-----------------
4 f
g
I
SRC
· f
, the higher is the gate charge Q
SW
SW
P
R
1
ON
SENSE 5
+
@I = I
T
LPK
–
-------------- m
I
L5 PK
50
g
moves from the input source to
(BR)DSS
must be greater than
g
, which leads to a
RRM
great-