LTC1700 Datasheet, PDF (8/16 Pages) Linear Technology – No RSENSE Synchronous Step-Up DC/DC Controller

LTC1700
APPLICATIONS INFORMATION
Power MOSFET Selection
The LTC1700 requires two external power MOSFETs, one
for the main switch (N-channel) and one for the synchro-
nous rectifier (P-channel). Since the voltage operating
range of the LTC1700 is limited to less than 6V, the
breakdown voltage of the MOSFETs is not a concern.
Therefore the MOSFETs parameters that should be used
for selecting the power MOSFETs are threshold voltage
V_GS(TH), on-resistance R_DS(ON), reverse transfer capaci-
tance C_RSS and maximum current I_D(MAX).
The gate drive voltage is set by the output voltage, V_OUT.
Since the LTC1700 exits the start-up mode at 2.3V, sub-
logic level threshold MOSFETs should be used in LTC1700
applications. Newer MOSFETs with guaranteed R_DSON at
gate voltage of 1.8V are now available and will work very
well with the LTC1700.
The MOSFETs on-resistance is chosen based on the
required load current. The maximum average output
current I_O(MAX) is :
I_O(MAX) = (I_PK – 0.5∆I)(1 – DC)
where:
I_PK = Peak Inductor Current
∆I = Inductor Ripple Current
DC = Duty Cycle
The peak inductor current is inherently limited in a
current mode controller. The maximum V_DS sense volt-
age of the main MOSFET is limited to 78mV. The LTC1700
will not allow peak inductor current to exceed 78mV/
R_{DS(ON)(N-CHANNEL)}. The following equation is a good
guide for determining the required R_DS(ON)(MAX), allow-
ing some margin for ripple current, current limit and
variations in the LTC1700 and external component val-
ues:
( ) ^RDS(ON)(MAX)
≅
∆V_SENSE
 _IO(MAX)
 ^{1– DC}
+
1
2
∆I_L


ρ_T
For 25°C operating condition, set ∆V_SENSE = 65mV. For
conditions that vary over the full temperature range, set
∆V_SENSE = 55mV.
The ρ_T is a normalized term accounting for the significant
variation in R_DS(ON) with temperature, typically about
0.375%/°C as shown in Figure 2. Junction to case tem-
perature T_JC is around 10°C in most applications. For a
maximum ambient temperature of 70°C, using ρ_80°C ≅ 1.2
in the above equation is a reasonable choice. This equation
is plotted in Figure 3 to illustrate the dependence of
maximum output current on R_DS(ON), assuming
∆I = 0.4I_O(MAX).
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
1700 F02
Figure 2. R_DS(ON) vs Temperature
5.0
4.5
4.0
3.5
DUTY CYCLE = 10%
3.0
2.5
2.0
DUTY CYCLE = 50%
1.5
DUTY CYCLE = 80%
1.0
0.5
0
0 10 20 30 40 50 60 70 80 90 100
R_DS(ON) (mΩ)
1700 F03
Figure 3. Maximum Current vs R_DS(ON)
Power dissipated by the main and synchronous
MOSFETs depends upon their respective duty cycles and
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