LTC3808 Datasheet, PDF (13/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
The typical LTC3808 application circuit is shown on
Figure 11. External component selection for the controller
is driven by the load requirement and begins with the
selection of the inductor and the power MOSFETs.
Power MOSFET Selection
The LTC3808’s controller requires two external power
MOSFETs: a P-channel MOSFET for the topside (main)
switch and a N-channel MOSFET for the bottom (synchro-
nous) switch. 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 the input supply voltage. Since the
LTC3808 is designed for operation down to low input
voltages, a sublogic level 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 LTC3808 is less than the
absolute maximum MOSFET V_GS rating, which is typically
8V.
The P-channel 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
LTC3808’s current comparator monitors the drain-to-
source voltage V_DS of the top P-channel MOSFET, which
is sensed between the SENSE⁺ 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 ∆V_SENSE(MAX) to
approximately 125mV when IPRG is floating (85mV when
IPRG is tied low; 204mV when IPRG is tied high).
The output current that the LTC3808 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
required R_DS(ON)MAX at 25°C (manufacturer’s specifica-
tion), allowing some margin for variations in the LTC3808
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 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 maxi-
mum ambient temperature of 70°C, using ρ_80°C ~ 1.3 in
the above equation is a reasonable choice.
The N-channel MOSFET’s on resistance is chosen based
on the short-circuit current limit (I_SC). The LTC3808’s
short-circuit current limit comparator monitors the drain-
to-source voltage V_DS of the bottom N-channel MOSFET,
3808f
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