LTC3780EG Datasheet, PDF (18/30 Pages) Linear Technology – High Efficiency, Synchronous,4-Switch Buck-Boost Controller

LTC3780
APPLICATIONS INFORMATION
where ∆I_L is peak-to-peak inductor ripple current. In buck
mode, the maximum average load current is:
I_{O U T(M AX,BU CK )}
=
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
2 •130mV
•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
160
150
140
130
120
110
100
0.1
1
V_IN/V_OUT (V)
10
3780 F08
Figure 8. Load Current vs V_IN/V_OUT
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 – 1}
V_OUT
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) = IOUT(MAX) • VOUT – VIN(MIN) V
C_OUT • V_OUT • f
( ) Ripple (Buck,Cap) = IOUT(MAX) • VIN(MAX) – VOUT V
C_OUT • V_IN(MAX) • f
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.
3780ff
18
For more information www.linear.com/LTC3780