Datasheet

13
LTC3830/LTC3830-1
3830fa
The LTC3830/LTC3830-1 overcomes this problem by
sensing the PV
CC1
voltage when G1 is high. If PV
CC1
is less
than (V
CC
+ 2.5V), the maximum G1 duty cycle is reduced
to 70% by clamping the COMP pin at 1.8V (QC in BLOCK
DIAGRAM). This increases the G2 on time and allows the
charge pump capacitor to be refreshed.
For Applications using an external supply to power PV
CC1
,
this supply must also be higher than V
CC
by at least 2.5V
to insure 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.
Figure 12 shows a typical 5V to 3.3V application using a
doubling charge pump to generate PV
CC1
.
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC3830 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 LTC3830’s operating frequency, but is generally
less than 50mA.
LTC3830 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
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 LTC3830 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
==
==
APPLICATIO S I FOR ATIO
WUUU
LTC3830
3830 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