Datasheet

LTC3822-1
13
38221f
APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are:
P
V
V
IRV
TOP
OUT
IN
OUT MAX T DS ON IN
=+••• •
•
() ()
22
2ρ
IICf
P
VV
V
I
OUT MAX RSS
BOT
IN OUT
IN
OUT MAX
()
()
••
–
•=
22
••
()
ρ
TDSON
R
Both MOSFETs have I
2
R losses and the P
TOP
equation
includes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest at high input voltage or during a short-circuit
when the bottom duty cycle is 100%.
The LTC3822-1 utilizes a non-overlapping, anti-shoot-
through gate drive control scheme to ensure that the
MOSFETs are not turned on at the same time. To function
properly, the control scheme requires that the MOSFETs
used are intended for DC/DC switching applications. Many
power MOSFETs are intended to be used as static switches
and therefore are slow to turn on or off.
Reasonable starting criteria for selecting the MOSFETs
are that they must typically have a gate charge (Q
G
) less
than 30nC (at 2.5V
GS
) and a turn-off delay (t
D(OFF)
) of less
than approximately 140ns. However, due to differences
in test and specifi cation methods of various MOSFET
manufacturers, and in the variations in Q
G
and t
D(OFF)
with
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
38221 F02
0.5
0
0
50
100
2.0
Figure 2. R
DS(ON)
vs Temperature
gate drive (V
IN
) voltage, the MOSFETs ultimately should
be evaluated in the actual LTC3822-1 application circuit
to ensure proper operation.
Shoot-through between the MOSFETs can most easily
be spotted by monitoring the input supply current. As
the input supply voltage increases, if the input supply
current increases dramatically, then the likely cause is
shoot-through.
Run and Soft-Start/Tracking Functions
The LTC3822-1 has a low power shutdown mode which is
controlled by the RUN pin. Pulling the RUN pin below 1.1V
puts the LTC3822-1 into a low quiescent current shutdown
mode (I
Q
= 7.2µA). Releasing the RUN pin, an internal
0.7µA (at V
IN
= 3.3V) current source will pull the RUN pin
up to V
IN
, which enables the controller. The RUN pin can
be driven directly from logic as showed in Figure 3.
3.3V
38221 F03
LTC3822-1
RUN
LTC3822-1
RUN
Figure 3. RUN Pin Interfacing
Once the controller is enabled, the start-up of V
OUT
is con-
trolled by the state of the TRACK/SS pin. If the TRACK/SS
pin is connected to V
IN
, the start-up of V
OUT
is controlled
by internal soft-start, which slowly ramps the positive
reference to the error amplifi er from 0V to 0.6V, allowing
V
OUT
to rise smoothly from 0V to its fi nal value. The de-
fault internal soft-start time is around 1ms. The soft-start
time can be changed by placing a capacitor between the
TRACK/SS pin and GND. In this case, the soft-start time
will be approximately:
tC
mV
A
SS SS
=
µ
•
600
1
where 1µA is an internal current source which is always
on.
When the voltage on the TRACK/SS pin is less than the
internal 0.6V reference, the LTC3822-1 regulates the V
FB
voltage to the TRACK/SS pin voltage instead of 0.6V.
Therefore the start-up of V
OUT
can ratiometrically track