LTC3772_15 Datasheet, PDF (10/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

LTC3772
APPLICATIO S I FOR ATIO
The basic LTC3772 application circuit is shown on the front
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of the power MOSFET inductor and the output diode.
These are selected followed by the input bypass capacitor
C_IN and output bypass capacitor C_OUT.
Power MOSFET Selection
An external P-channel power MOSFET must be selected for
use with the LTC3772. The main selection criteria for the
power MOSFET are the threshold voltage V_GS(TH) and the
“on” resistance R_DS(ON), reverse transfer capacitance C_RSS
and total gate charge.
Since the LTC3772 is designed for operation down to low
input voltages, a sublogic level threshold MOSFET (R_DS(ON)
guaranteed at V_GS = 2.5V) is required for applications that
work close to this voltage. When these MOSFETs are used,
make sure that the input supply to the LTC3772 is less than
the absolute maximum V_GS rating.
The P-channel MOSFET’s on-resistance is chosen based on
the required load current. The maximum average output
load current I_OUT(MAX) is equal to the peak inductor current
minus half the peak-to-peak ripple current I_RIPPLE. The
LTC3772’s current comparator monitors the drain-to-
source voltage V_DS of the P-channel MOSFET, which is
sensed between the V_IN and SW pins. The peak inductor
current is limited by the current threshold, set by the volt-
age on the I_TH pin of the current comparator. The voltage
on the I_TH pin is internally clamped, which limits the maxi-
mum current sense threshold ∆V_SENSE(MAX) to approxi-
mately 175mV when I_PRG is floating (100mV when I_PRG is
tied low; 250mV when I_PRG is tied high).
The output current that the LTC3772 can provide is given by:
I_OUT(MAX)
=
∆V_SENSE(MAX)
R_DS(ON)
–
I_RIPPLE
2
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%.
10
However, for operation above 20% duty cycle, slope com-
pensation 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
•
∆V_SENSE(MAX)
I_OUT(MAX)
–
SF
where SF is a 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
required R_DS(ON)MAX at 25°C (manufacturer’s specifica-
tion), allowing some margin for variations in the LTC3772
and external component values:
R_DS(ON)(MAX)
=
5
6
•
0.9
•
∆V_SENSE(MAX) – SF
I_OUT(MAX) • ρ_T
The ρ_T is a normalizing term accounting for the tempera-
ture variation in on-resistance, which is typically about
0.4%/°C, as shown in Figure 2. Junction to case tempera-
ture T_JC is about 10°C in most applications. For a maximum
ambient temperature of 70°C, using ρ_80°C ~ 1.3 in the above
equation is a reasonable choice.
The required minimum R_DS(ON) of the MOSFET is also
governed by its allowable power dissipation. For applica-
tions that may operate the LTC3772 in dropout–i.e., 100%
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (ϒC)
3772 F02
Figure 2. R_DS(ON) vs Temperature
3772f