LTC3822-1_15 Datasheet, PDF (12/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous Step-Down DC/DC Controller

LTC3822-1
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 floating (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_OUT(MAX)
=
∆V_SENSE(MAX)
R_DS(ON)
–
I_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_DS(ON)MAX
=
5
6
•
∆V_SENSE(MAX)
I_OUT(MAX)
for Duty Cycle < 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:
R_DS(ON)MAX
=
5
6
•
SF
•
∆V_SENSE(MAX)
I_OUT(MAX)
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
significant 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 specification),
allowing some margin for variations in the LTC3822-1 and
external component values:
R_DS(ON)MAX
=
5
6
•
0.9
•
SF
•
∆V_SENSE(MAX)
I_OUT(MAX) • ρ_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 = ^VOUT
V_IN
Bottom MOSFET Duty Cycle = ^{VIN – VOUT}
V_IN
38221f
12