Datasheet

LTC3822-1
12
38221f
APPLICATIONS INFORMATION
The typical LTC3822-1 application circuit is shown on
the front page of this data sheet. External component
selection for the controller is driven by the load require-
ment and begins with the selection of the inductor and
the power MOSFETs.
Power MOSFET Selection
The LTC3822-1’s controller requires external N-chan-
nel power MOSFETs for the topside (main) and bottom
(synchronous) switches. The main selection criteria for
the power MOSFETs are the breakdown voltage V
BR(DSS)
,
threshold voltage V
GS(TH)
, on-resistance R
DS(ON)
, reverse
transfer capacitance C
RSS
, turn-off delay t
D(OFF)
and the
total gate charge Q
G
.
The gate drive voltage is usually the input supply voltage.
Since the LTC3822-1 is designed for operation at low input
voltages, a sublogic level MOSFET (R
DS(ON)
guaranteed at
V
GS
= 2.5V) is required.
The topside MOSFET’s on-resistance is chosen based on
the required load current. The maximum average load
current I
OUT(MAX)
is equal to the peak inductor current
minus half the peak-to-peak ripple current I
RIPPLE
. The
LTC3822-1’s current comparator monitors the drain-to-
source voltage V
DS
of the top MOSFET, which is sensed
between the V
IN
and SW pins. The peak inductor current
is limited by the current threshold, set by the voltage on
the I
TH
pin, of the current comparator. The voltage on the
I
TH
pin is internally clamped, which limits the maximum
current sense threshold ΔV
SENSE(MAX)
to approximately
125mV when IPRG is fl oating (82mV when IPRG is tied
low; 200mV when IPRG is tied high).
The output current that the LTC3822-1 can provide is
given by:
I
V
R
I
OUT MAX
SENSE MAX
DS ON
RIPPLE
()
()
()
–=
∆
2
where I
RIPPLE
is the inductor peak-to-peak ripple current
(see Inductor Value Calculation).
A reasonable starting point is setting ripple current I
RIPPLE
to be 40% of I
OUT(MAX)
. Rearranging the above equation
yields:
R
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
•=
5
6
∆
for DutyyCycle<20%
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R
DS(ON)
to provide the required
amount of load current:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
••=
5
6
∆
where SF is a scale factor whose value is obtained from
the curve in Figure 1.
These must be further derated to take into account the
signifi cant variation in on-resistance with temperature. The
following equation is a good guide for determining the re-
quired R
DS(ON)MAX
at 25°C (manufacturer’s specifi cation),
allowing some margin for variations in the LTC3822-1 and
external component values:
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
•.• •
•
=
5
6
09
∆
ρρ
T
The ρ
T
is a normalizing term accounting for the temperature
variation in on-resistance, which is typically about 0.4%/°C,
as shown in Figure 2. Junction-to-case temperature ΔT
JC
is about 10°C in most applications. For a maximum ambi-
ent temperature of 70°C, using ρ
80°C
≈ 1.3 in the above
equation is a reasonable choice.
The power dissipated in the MOSFETs strongly depends
on their respective duty cycles and load current. When
the LTC3822-1 is operating in continuous mode, the duty
cycles for the MOSFETs are:
Top MOSFET Duty Cycle =
V
OUT
V
Bottom MOSFET
IN
–
Duty Cycle
VV
V
IN OUT
IN
=