LTC3832 Datasheet, PDF (14/24 Pages) Linear Technology – High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation

LTC3832/LTC3832-1
APPLICATIO S I FOR ATIO
Note that the required R_DS(ON) for Q2 is roughly three
times that of Q1 in this example. Note also that while the
required R_DS(ON) values suggest large MOSFETs, the
power dissipation numbers are only 0.83W per device or
less; large TO-220 packages and heat sinks are not neces-
sarily required in high efficiency applications. Siliconix
Si4410DY or International Rectifier IRF7413 (both in
SO-8) or Siliconix SUD50N03-10 (TO-252) or ON Semi-
conductor MTD20N03HDL (DPAK) are small footprint
surface mount devices with R_DS(ON) values below 0.03Ω
at 5V of V_GS that work well in LTC3832 circuits. Using a
higher P_MAX value in the R_DS(ON) calculations generally
decreases the MOSFET cost and the circuit efficiency and
increases the MOSFET heat sink requirements.
Table 1 highlights a variety of power MOSFETs for use in
LTC3832 applications.
Inductor Selection
The inductor is often the largest component in an LTC3832
design and must be chosen carefully. Choose the inductor
value and type based on output slew rate requirements. The
maximum rate of rise of inductor current is set by the
inductor’s value, the input-to-output voltage differential and
the LTC3832’s maximum duty cycle. In a typical 3.3V in-
put, 2.5V output application, the maximum rise time will be:
DC_MAX • (V_IN – V_OUT) ₌ 0.76 A
L_O
L_O µs
where L_O is the inductor value in µH. With proper fre-
quency compensation, the combination of the inductor
and output capacitor values determine the transient recov-
ery time. In general, a smaller value inductor improves
transient response at the expense of ripple and inductor
core saturation rating. A 1µH inductor has a 0.76A/µs rise
time in this application, resulting in a 6.6µs delay in
responding to a 5A load current step. During this 6.6µs,
the difference between the inductor current and the output
current is made up by the output capacitor. This action
causes a temporary voltage droop at the output. To
minimize this effect, the inductor value should usually be
in the 1µH to 5µH range for most 3.3V input LTC3832
circuits. To optimize performance, different combinations
of input and output voltages and expected loads may
require different inductor values.
Once the required value is known, the inductor core type
can be chosen based on peak current and efficiency
Table 1. Recommended MOSFETs for LTC3832 Applications
PARTS
Siliconix SUD50N03-10
TO-252
R_DS(ON)
AT 25°C (mΩ)
19
RATED CURRENT (A)
15 at 25°C
10 at 100°C
TYPICAL INPUT
CAPACITANCE
C_ISS (pF)
3200
Siliconix Si4410DY
SO-8
20
10 at 25°C
2700
8 at 70°C
ON Semiconductor MTD20N03HDL
35
20 at 25°C
880
DPAK
16 at 100°C
Fairchild FDS6670A
S0-8
8
13 at 25°C
3200
Fairchild FDS6680
SO-8
10
11.5 at 25°C
2070
ON Semiconductor MTB75N03HDL
9
DD PAK
75 at 25°C
4025
59 at 100°C
IR IRL3103S
DD PAK
19
64 at 25°C
1600
45 at 100°C
IR IRLZ44
TO-220
28
50 at 25°C
3300
36 at 100°C
Fuji 2SK1388
TO-220
37
35 at 25°C
1750
Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information.
14
θ_JC (°C/W)
1.8
1.67
25
25
1
1.4
1
2.08
T_JMAX (°C)
175
150
150
150
150
150
175
175
150
sn3832 3832fs