Datasheet

LTC3780
18
3780ff
For more information www.linear.com/LTC3780
applicaTions inForMaTion
where ∆I
L
is peak-to-peak inductor ripple current. In buck
mode, the maximum average load current is:
I
OUT(MAX,BUCK)
=
130mV
R
SENSE
+
∆I
L
2
Figure 8 shows how the load current (I
MAXLOAD
• R
SENSE
)
varies with input and output voltage
The maximum current sensing R
SENSE
value for the boost
mode is:
R
SENSE(MAX)
=
2•160mV • V
IN(MIN)
2•I
OUT(MAX,BOOST)
• V
OUT
+ ∆I
L,BOOST
• V
IN(MIN)
The maximum current sensing R
SENSE
value for the buck
mode is:
R
SENSE(MAX)
=
2• 130mV
2•I
OUT(MAX,BUCK)
– ∆I
L,BUCK
The final R
SENSE
value should be lower than the calculated
R
SENSE(MAX)
in both the boost and buck modes. A 20% to
30% margin is usually recommended.
C
IN
and C
OUT
Selection
In boost mode, input current is continuous. In buck mode,
input current is discontinuous. In buck mode, the selection
of input capacitor C
IN
is driven by the need to filter the
input square wave current. Use a low ESR capacitor sized
to handle the maximum RMS current. For buck operation,
the input RMS current is given by:
I
RMS
≈I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
– 1
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT(MAX)
/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple cur
-
rent ratings from capacitor manufacturers are often based
on only 2000 hours of life which makes it advisable to
derate the capacitor.
In boost mode, the discontinuous current shifts from the
input to the output, so C
OUT
must be capable of reducing
the output voltage ripple. The effects of ESR (equivalent
series resistance) and the bulk capacitance must be
considered when choosing the right capacitor for a given
output ripple voltage. The steady ripple due to charging
and discharging the bulk capacitance is given by:
Ripple (Boost,Cap) =
I
OUT(MAX)
• V
OUT
– V
IN(MIN)
(
)
C
OUT
• V
OUT
• f
V
Ripple (Buck,Cap) =
I
OUT(MAX)
• V
IN(MAX)
– V
OUT
(
)
C
OUT
• V
IN(MAX)
• f
V
where C
OUT
is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
∆V
BOOST,ESR
= I
L(MAX,BOOST)
• ESR
∆V
BUCK,ESR
= I
L(MAX,BUCK)
• ESR
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings, such as OS-CON and POSCAP.
V
IN
/V
OUT
(V)
0.1
100
I
MAX(LOAD)
• R
SENSE
(mV)
110
120
130
140
160
1 10
3780 F08
150
Figure 8. Load Current vs V
IN
/V
OUT