LTC3831-1 Linear Technology, LTC3831-1 Datasheet - Page 13

• Manufacturer Part Number: LTC3831-1
• Description: High Power Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC3831-1 circuits. These should be selected based pri-
marily on threshold voltage and on-resistance consider-
ations. Thermal dissipation is often a secondary concern
in high efficiency designs. The required MOSFET thresh-
old should be determined based on the available power
supply voltages and/or the complexity of the gate drive
charge pump scheme. In 3.3V input designs where an
auxiliary 12V supply is available to power PV
standard MOSFETs with R
6V can be used with good results. The current drawn from
this supply varies with the MOSFETs used and the LTC3831-
1’s operating frequency, but is generally less than 50mA.
LTC3831-1 applications that use 5V or lower V
and tripling charge pumps to generate PV
do not provide enough gate drive voltage to fully enhance
standard power MOSFETs. Under this condition, the effec-
tive MOSFET R
dissipation in the FETs and reducing efficiency. Logic-
level or sub-logic level FETs are the recommended choice
for 5V or lower voltage systems. Logic-level FETs can be
fully enhanced with a tripling charge pump and will oper-
ate at maximum efficiency.
3.3V
2.2 F
DS(ON)
C1
33pF
U
may be quite high, raising the
4.7 F
SHDN
U
R
1k
DS(ON)
C
C
10nF
0.01 F
C
specified at V
V
SS
FREQSET
SHDN
COMP
PV
CC
W
LTC3831-1
CC2
Figure 6. Typical Application with V
R
–
PV
CC1
12V
PGND
CC1
I
GND
CC1
MAX
TG
I
BG
R
FB
FB
+
and PV
and PV
U
GS
IN
= 5V or
voltage
CC2
CC2
10k
C
C
Q1, Q2: SILICONIX Si4410DY
,
,
IN
OUT
: SANYO POSCAP 4TPB220M
: SANYO POSCAP 4TPB470M
After the MOSFET threshold voltage is selected, choose
the R
allowable power dissipation and maximum output cur-
rent. In a typical LTC3831-1 circuit operating in continu-
ous mode, the average inductor current is equal to the
output load current. This current flows through either Q1
or Q2 with the power dissipation split up according to the
duty cycle:
The R
be calculated by rearranging the relation P = I
P
efficiency or allowable thermal dissipation. A typical high
R
R
MAX
DS ON Q
DS ON Q
0.1 F
1k
DC Q
DC Q
(
(
DS(ON)
DS(ON)
( )
(
should be calculated based primarily on required
TT
)
)
1
2
= 0.6 • V
1
2
)
V
based on the input voltage, the output voltage,
1.5V
required for a given conduction loss can now
DDQ
V
1
DC Q
DC Q
Q2
Q1
V
OUT
–
IN
( ) • (
(
V
DDQ
P
V
P
OUT
MAX Q
1
IN
MAX Q
2
) • (
MBRS340T3
MBRS340T3
1.2 H
L
( )
I
O
(
LOAD
I
LOAD
1
2
V
)
IN
+
)
–
V
)
2
2
IN
V
C
470 F
OUT
OUT
V
(
V
OUT
+
V
IN
IN
LTC3831-1
2k
1%
10k
1%
38311 F06
•
V
–
C
220 F
V
(V
0.9V
P
• (
IN
TT
IN
6A
OUT
MAX Q
V
I
LOAD
OUT
•
)
P
( )
MAX Q
2
) • (
1
R.
)
2
(
13
I
LOAD
2
)
38311f
)
2