Datasheet

LTC1700
8
1700fa
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:
R
V
I
DC
I
DS ON MAX
SENSE
OMAX
LT
()( )
()
–
≅
∆
+
⎛
⎝
⎜
⎞
⎠
⎟
()
1
1
2
∆ρ
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)
.
APPLICATIONS INFORMATION
WUU
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Figure 2. R
DS(ON)
vs Temperature
Figure 3. Maximum Current vs R
DS(ON)
Power dissipated by the main and synchronous
MOSFETs depends upon their respective duty cycles and
R
DS(ON)
(mΩ)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
MAXIMUM OUTPUT CURRENT (A)
1700 F03
0 102030
40
50
60 70 80 90 100
DUTY CYCLE = 10%
DUTY CYCLE = 50%
DUTY CYCLE = 80%
TEMPERATURE (°C)
–55
ρ
T
NORMALIZED ON RESISTANCE
65
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
1700 F02
–35 125
–15
52545 85
105