LTC3852
24
3852f
Further input noise reduction can be achieved by powering
V
IN1
through a very small series inductor as shown in
Figure 12. A 10nH inductor will reject the fast current
notches, thereby presenting a nearly constant current
load to the input power supply. For economy, the 10nH
inductor can be fabricated on the PC board with about
1cm (0.4") of PC board trace.
APPLICATIONS INFORMATION
LTC3852
0.22µF
2.2µF
V
IN1
GND1
1cm OF PCB TRACE
10nH
V
IN
11
12
3852 F12
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the fl ying capacitor
since its voltage can reverse upon start-up of the
charge pump. Low ESR ceramic capacitors should always
be used for the fl ying capacitor.
The fl ying capacitor controls the strength of the charge
pump. In order to achieve the rated output current, it is
necessary to have at least 1µF of capacitance for the fl y ing
capacitor.
Ceramic Capacitors
Ceramic capacitors of different materials lose their capac-
itance with higher temperature and voltage at different rates.
For example, a capacitor made of X5R or X7R material
will retain most of its capacitance from –40°C to 85°C
whereas a Z5U or Y5V style capacitor will lose considerable
capacitance over that range. Z5U and Y5V capacitors
may also have a poor voltage coeffi cient causing them
to lose 60% or more of their capacitance when the rated
voltage is applied. Therefore when comparing different
capacitors, it is often more appropriate to compare the
amount of achievable capacitance for a given case size
rather than discussing the specifi ed capacitance value. For
example, over rated voltage and temperature conditions,
a 1µF 10V Y5V ceramic capacitor in a 0603 case may not
Figure 12. 10nH Inductor Used for
Additional Input Noise Reduction
provide any more capacitance than a 0.22µF 10V X7R
capacitor available in the same 0603 case. In fact, for the
charge pump, these capacitors can be considered roughly
equivalent. The capacitor manufacturer’s data sheet should
be consulted to ensure the desired capacitance at all temp-
eratures and voltages.
Table 1 shows a list of ceramic capacitor manufacturers
and how to contact them:
Table 1.
AVX www.avx.com
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
TDK www.component.tdk.com
Vishay www.vishay.com
Effi ciency Considerations
The percent effi ciency of a switching regulator is equal to
the output power divided by the input power times 100. It
is often useful to analyze the individual loss components
to determine what limits the effi ciency and which change
would produce the biggest improvement. The effi ciency
can be expressed as:
% Effi ciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, there are fi ve main sources of power loss in
LTC3851 circuits: 1) I
2
R losses, 2) transition losses in the
top MOSFET, 3) gate charge losses within the controller
due to the input capacitance of the power MOSFETs,
4) the DC bias current of the controller (V
IN2
), and 5) the
effi ciency of the charge pump.
1. I
2
R losses are predicted from the DC resistances of the
fuse (if used), top and bottom MOSFET on-resistances,
the inductor DCR and the current sense resistor (if used).
In continuous conduction mode (CCM), the average
output current fl ows through the inductor (L) and sense
resistor (R
SENSE
), but is “chopped” between the top
and bottom MOSFETs. Since the two MOSFETs rarely
have the same R
ON
, an effective MOSFET resistance
