LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 17

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
where ∆I
mode, the maximum average load current is:
Figure 8 shows how the load current (I
varies with input and output voltage
The maximum current sensing R
mode is:
The maximum current sensing R
mode is:
The fi nal R
R
30% margin is usually recommended.
C
In boost mode, input current is continuous. In buck mode,
input current is discontinuous. In buck mode, the selection
of input capacitor C
input square wave current. Use a low ESR capacitor sized
IN
SENSE(MAX)
I
R
R
OUT(MAX,BUCK)
2 I
and C
SENSE(MAX)
SENSE(MAX)
OUT(MAX,BOOST)
L
OUT
160
140
100
is peak-to-peak inductor ripple current. In buck
SENSE
150
130
120
110
0.1
in both the boost and buck modes. A 20% to
Figure 8. Load Current vs V
Selection
=
2 160mV V
=
value should be lower than the calculated
2 I
=
IN
130mV
R
OUT(MAX,BUCK)
SENSE
is driven by the need to fi lter the
V
V
OUT
IN
/V
2 130mV
OUT
1
+ ΔI
IN(MIN)
+
(V)
ΔI
SENSE
2
SENSE
L,BOOST
L
– ΔI
IN
MAXLOAD
value for the boost
/V
value for the buck
3780 F08
OUT
L,BUCK
V
10
IN(MIN)
• R
SENSE
)
to handle the maximum RMS current. For buck operation,
the input RMS current is given by:
This formula has a maximum at V
I
is commonly used for design because even signifi cant
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
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:
where C
The steady ripple due to the voltage drop across the ESR
is given by:
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 coeffi cient.
Capacitors are now available with low ESR and high ripple
current ratings, such as OS-CON and POSCAP .
RMS
∆V
∆V
I
Ripple (Boost,Cap) =
Ripple (Buck,Cap) =
RMS
BOOST,ESR
BUCK,ESR
= I
OUT
≈ I
OUT(MAX)
OUT(MAX)
is the output fi lter capacitor.
= I
= I
L(MAX,BUCK)
/2. This simple worst-case condition
L(MAX,BOOST)
•
V
V
OUT
I
OUT
OUT(MAX)
IN
I
OUT(MAX)
must be capable of reducing
•
C
• ESR
OUT
• ESR
V
C
V
OUT
OUT
IN
• V
• V
(
• V
(
IN
– 1
• V
IN(MAX )
IN(MAX)
OUT
LTC3780
= 2V
OUT
– V
• f
OUT
– V
• f
IN(MIN)
, where
OUT
17
3780fe
)
)
V
V