LTC3772B Datasheet, PDF (10/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

LTC3772B
APPLICATIO S I FOR ATIO
The basic LTC3772B application circuit is shown on the front
page of this data sheet. The load requirement drives the
selection of external components: the power MOSFET,
inductor and output diode, as well as 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 LTC3772B. The main selection criteria for the
power MOSFET are the threshold voltage V_GS(TH), the “on”
resistance R_DS(ON), reverse transfer capacitance C_RSS and
total gate charge.
Since the LTC3772B 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 LTC3772B 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
LTC3772B’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 138mV when I_PRG is floating (70mV when I_PRG is
tied low; 208mV when I_PRG is tied high).
The output current that the LTC3772B 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) for 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 LTC3772B
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 LTC3772B in dropout–i.e., 100%
2.0
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (ϒC)
3772B F02
Figure 2. R_DS(ON) vs Temperature
3772bfa