LTC3789 Datasheet, PDF (15/28 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
Applications Information
The Typical Application on the first page is a basic
LTC3789 application circuit. External component selec-
tion is driven by the load requirement, and begins with
the selection of R_SENSE and the inductor value. Next, the
power MOSFETs are selected. Finally, C_IN and C_OUT are
selected. This circuit can be configured for operation up
to an input voltage of 38V.
R_SENSE Selection and Maximum Output Current
R_SENSE is chosen based on the required output current.
The current comparator threshold sets the peak of the
inductor current in the boost region and the maximum
inductor valley current in the buck region. In the boost
region, the maximum average load current at V_IN(MIN) is:
I_{OUT(M AX ,BOOST )}
=
 _140mV
 R_SENSE
–
∆I_L
2


•
V_IN(MIN)
V_OUT
where ∆I_L is peak-to-peak inductor ripple current. In the
buck region, the maximum average load current is:
I_{OUT(MAX,BUCK )}
=
90mV
R_SENSE
+
∆I_L
2
Figure 6 shows how the load current (I_LOAD(MAX) • R_SENSE)
varies with input and output voltage.
160
150
140
130
120
110
100
90
0.1
1
V_IN/ V_OUT (V)
10
3789 F06
Figure 6. Load Current vs V_IN/V_OUT
The maximum current sensing R_SENSE value for the boost
region is:
R_SENSE(MAX) =
2 • 140mV • 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
region is:
R_{SENSE(MAX )}
=
2
•
2 • 90mV
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 regions. A 20%
to 30% margin is usually recommended.
Programming Input/Output Current Limit
As shown in Figures 7 and 8, current sense resistor R_SENSE2
should be placed between the buck capacitor for V_IN/V_OUT
and the decoupling capacitor. A lowpass filter formed by R_F
and C_F is recommended to reduce the switching noise. The
input/output current limit is set by the I_LIM pin for 50mV,
100mV or 140mV with I_LIM pulled to the GND, floating, or
tied to INTV_CC, respectively. If input/output current limit
is not desired, the I_OSENSE+ and I_OSENSE– pins should be
shorted to either V_OUT or V_IN.
FROM
CONTROLLER
V_OUT
R_SENSE2
R_F
C_F
100Ω
R_F ⁺
100Ω
TO
SYSTEM
V_OUT
2
I_OSENSE+
LTC3789
1
I_OSENSE–
3789 F07
Figure 7. Programming Output Current Limit
FROM DC
POWER INPUT ₊
R_SENSE2
R_F
C_F
100Ω
R_F
100Ω
TO REMAINDER
OF SYSTEM
2
I_OSENSE+
LTC3789
1
I_OSENSE–
3789 F08
Figure 8. Programming Input Current Limit
3789f
15