ltc3831egn-trpbf Linear Technology Corporation, ltc3831egn-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc3831egn-trpbf
• Description: High Power Synchronous Switching Regulator Controller For Ddr Memory Termination
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC3831 circuits. These should be selected based primarily
on threshold voltage and on-resistance considerations.
Thermal dissipation is often a secondary concern in high
effi ciency designs. The required MOSFET threshold should
be determined based on the available power supply volt-
ages 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
MOSFETs with R
be used with good results. The current drawn from this
supply varies with the MOSFETs used and the LTC3831’s
operating frequency, but is generally less than 50mA.
LTC3831 applications that use 5V or lower V
doubling/tripling charge pumps to generate PV
PV
enhance standard power MOSFETs. Under this condition,
the effective MOSFET R
the dissipation in the FETs and reducing effi ciency. Logic-
level FETs are the recommended choice for 5V or lower
voltage systems. Logic-level FETs can be fully enhanced
with a doubler/tripling charge pump and will operate at
maximum effi ciency.
CC2
, do not provide enough gate drive voltage to fully
+
DS(ON)
4.7μF
C1
33pF
0.1μF
DS(ON)
specifi ed at V
1μF
SHDN
R
15k
C
C
1500pF
0.01μF
130k
C
may be quite high, raising
CC1
V
SS
FREQSET
SHDN
COMP
PV
CC
CC2
and PV
LTC3831
Figure 8. Typical Application with V
GS
5V
R
–
PV
= 5V or 6V can
PGND
I
GND
CC1
MAX
CC2
IN
TG
I
BG
R
FB
FB
MBR0530T1
+
voltage and
, standard
CC1
and
10k
C
C
Q1, Q2: SILICONIX Si4410DY
IN
OUT
: SANYO POSCAP 6TPB330M
: SANYO POSCAP 4TPB470M
After the MOSFET threshold voltage is selected, choose
the R
allowable power dissipation and maximum output current.
In a typical LTC3831 circuit operating in continuous mode,
the average inductor current is equal to the output load
current. This current fl ows 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
effi ciency or allowable thermal dissipation. A typical high
effi ciency circuit designed for 2.5V input and 1.25V at 5A
R
R
MAX
0.1μF
1k
DS(ON)Q1
DS(ON)Q2
DC(Q1) =
DC(Q2) = 1–
DS(ON)
DS(ON)
should be calculated based primarily on required
TT
0.1μF
= 0.6 • V
=
V
=
2.5V
based on the input voltage, the output voltage,
DDQ
required for a given conduction loss can now
V
DC(Q1) • (I
Q2
DC(Q2) • (I
Q1
V
OUT
IN
V
DDQ
P
V
OUT
P
MAX(Q1)
IN
MAX(Q2)
MBRS340T3
MBRS340T3
1.2μH
L
=
O
LOAD
LOAD
V
IN
+
– V
)
V
)
2
IN
2
=
C
470μF
×3
OUT
=
OUT
V
(V
V
OUT
+
IN
IN
2k
1%
10k
1%
3831 F08
• P
V
– V
C
330μF
×2
• (I
V
1.5V
±6A
IN
IN
LTC3831
TT
MAX(Q1)
LOAD
OUT
• P
MAX(Q2)
) • (I
2
)
2
R.
LOAD
13
3831fa
)
2