LT1619 Linear Technology, LT1619 Datasheet - Page 8

• Manufacturer Part Number: LT1619
• Description: Low Voltage Current Mode PWM Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1619
Power MOSFET
MOSFET power dissipation can be separated into fre-
quency independent and frequency dependent compo-
nents. The R
mean square switch current and switch R
does not vary with the operating frequency.
The frequency-dependent switching losses consist of 1)
switch transition loss due to finite rise and fall times of the
drain source voltage and the drain current 2) gate switch-
ing loss, i.e., a packet of charge Q
which is moved from the gate drive power supply to
ground in every switch cycle, and 3) the drain switching
loss, charge stored on the parasitic drain capacitance,
C
transistor loss can be expressed as:
where the transition loss can be estimated with:
At low V
500kHz, the ohmic losses often dominate. For high voltage
converters, the transition loss and C
loss can dramatically impact the converter efficiency.
MOSFETs with lower parasitic capacitances but higher
R
situations.
Capacitors
In a switch mode DC/DC converter, output ripple voltage
is the product of the equivalent series resistance (ESR) of
8
OSS
DS(ON)
P
+ 1/2C
Q
V
I
f
C
Transition Loss I
G(AVG)
S
LOSS
G
RSS
g
= Operating frequency
is dumped to ground as the switch is turned on. The
= The total gate charge
= Gate drive voltage V
= The average C
DS(OFF)
may actually provide better efficiency in these
OSS
= I
= The average MOSFET buffer output current
and V
DS(ON)
DRMS
V
DS(OFF)
DS
( 12V) and operating frequencies below
2
= V
loss in the switch is the product of the
U
R
2
DS(ON)
f
D
DS(OFF)
S
C
GD
RSS DS OFF
U
between V
I
+ transition loss + Q
V
G AVG
DRV
(
(
g
(the total gate charge)
W
)
OSS
)
2
DS
f
S
charge dumping
= 0V
DS(ON)
U
and it
g
V
G
f
S
the output capacitor and the peak-to-peak capacitor
current. Depending on topology, current feeding the out-
put capacitor can be continuous or discontinuous. The input
current can also be continuous or discontinuous even if the
inductor current itself is continuous. In boost topology, the
inductor is in series with the input source so the input
current is continuous and the output current is discontinu-
ous. In buck-boost or flyback converters, the inductor is
not in series with the input source nor the output, so nei-
ther the input current nor output current is continuous.
Whenever a terminal current is discontinuous, the capaci-
tor at that terminal should be chosen to handle the ripple
current. Capacitor reliability will be adversely affected if
the ripple current exceeds the maximum allowable rat-
ings. This maximum rating is specified as the RMS ripple
current. Several capacitors may be mounted in parallel to
meet the size and ripple current requirements.
Besides the ripple voltage requirements, the output ca-
pacitor also needs to be sized for acceptable output
voltage variation under load transients.
Current Sensing Resistor R
The LT1619 drives a low side N-channel MOSFET switch.
The switch current is sensed with an external resistor
R
ground. The internal blanking circuit blocks the voltage
spike developed across R
on. The switch is turned off when the instantaneous
voltage across R
V
The current limit threshold is constant and does not vary
with duty ratio.
Due to low signal level of the sense voltage, low inductance
sense resistors are required to reduce switching noise.
Low TC resistors maintain constant current limit over
temperature. Dale WSL and IRC series sense resistors
meet these criteria.
SENSE
SENSE
R
SENSE
. Allowing variations in V
connected between the source of the MOSFET and
V
SENSE MIN
SENSE
I
L MAX
(
(
exceeds the current limit threshold,
)
)
SENSE
SENSE
for 280ns at switch turn-
SENSE
yields:
1619fa