Datasheet

13
LTC3832/LTC3832-1
sn3832 3832fs
Diagram). This increases the G2 on-time and allows the
charge pump capacitors to be refreshed.
For applications using an external supply to PV
CC1
, this
supply must also be higher than V
CC
by at least 2.5V to
ensure normal operation.
For applications with a 5V or higher V
IN
supply, PV
CC2
can
be tied to V
IN
if a logic level MOSFET is used. PV
CC1
can be
supplied using a doubling charge pump as shown in
Figure␣ 9. This circuit provides 2V
IN
– V
F
to PV
CC1
while Q1
is ON.
enhance standard power MOSFETs. Under this condition,
the effective MOSFET R
DS(ON)
may be quite high, raising
the dissipation in the FETs and reducing efficiency. 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 efficiency.
After the MOSFET threshold voltage is selected, choose the
R
DS(ON)
based on the input voltage, the output voltage,
allowable power dissipation and maximum output current.
In a typical LTC3832 circuit, operating in continuous 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:
DC Q
V
V
DC Q
V
V
VV
V
OUT
IN
OUT
IN
IN OUT
IN
()
() –
–
1
21
=
==
The R
DS(ON)
required for a given conduction loss can now
be calculated by rearranging the relation P = I
2
R.
R
P
DC Q I
VP
VI
R
P
DC Q I
VP
VV I
DS ON Q
MAX Q
LOAD
IN MAX Q
OUT LOAD
DS ON Q
MAX Q
LOAD
IN MAX Q
IN OUT LOAD
()
() ()
()
() ()
()•( )
•
•( )
()•( )
•
(– )•( )
1
1
2
1
2
2
2
2
2
2
1
2
==
==
P
MAX
should be calculated based primarily on required
efficiency or allowable thermal dissipation. A typical high
efficiency circuit designed for 3.3V input and 2.5V at 10A
output might allow no more than 3% efficiency loss at full
load for each MOSFET. Assuming roughly 90% efficiency
at this current level, this gives a P
MAX
value of:
(2.5V)(10A/0.9)(0.03) = 0.83W per FET
and a required R
DS(ON)
of:
R
VW
VA
R
VW
VVA
DS ON Q
DS ON Q
()
()
(. )•(. )
( . )( )
.
(. )•(. )
( . – . )( )
.
1
2
2
2
33 083
25 10
0 011
33 083
33 25 10
0 034
==Ω
==Ω
APPLICATIO S I FOR ATIO
WUUU
LTC3832
3832 F09
+
D
Z
12V
1N5242
Q1
L
O
Q2 C
OUT
V
OUT
0.1µF
PV
CC2
OPTIONAL
USE FOR V
IN
≥ 7V
MBR0530T1
PV
CC1
G1
G2
V
IN
Figure 9. Doubling Charge Pump
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC3832 circuits. These should be selected based
primarily on threshold voltage and on-resistance consid-
erations. Thermal dissipation is often a secondary con-
cern in high efficiency designs. The required MOSFET
threshold 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
CC1
and
PV
CC2
, standard MOSFETs with R
DS(ON)
specified at V
GS
= 5V or 6V can be used with good results. The current
drawn from this supply varies with the MOSFETs used
and the LTC3832’s operating frequency, but is generally
less than 50mA.
LTC3832 applications that use 5V or lower V
IN
voltage and
a doubling/tripling charge pump to generate PV
CC1
and
PV
CC2
, do not provide enough gate drive voltage to fully