LM34919C-Q1 Datasheet, PDF (20/29 Pages) Texas Instruments – LM34919C-Q1 Ultra Small 50V, 600 mA Constant On-Time Buck Switching Regulator

LM34919C-Q1
SNVS831A – SEPTEMBER 2013 – REVISED DECEMBER 2013
LM34919C
L1
SW
Cff
FB
V_OUT
R1
R3
R2
C2
www.ti.com
Figure 12. Reduced Ripple Configuration
b) Minimum ripple configuration: The circuit of Figure 13 provides minimum ripple at V_OUT, determined
primarily by characteristics of C2 and the inductor’s ripple current since R3 is removed. RA and CA are chosen to
generate a sawtooth waveform at their junction and that voltage is AC-coupled to the FB pin via CB. To
determine the values for RA, CA and CB, use the following procedure:
Calculate V_A = V_OUT - (V_SW x (1 - (V_OUT/V_IN(min))))
(17)
where V_SW is the absolute value of the voltage at the SW pin during the off-time (typically 1 V). V_A is the DC
voltage at the RA/CA junction. Calculate the RA-CA product in Equation 18.
(V_IN(min) - V_A) x t_ON
RA x CA =
V
(18)
where t_ON is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the
RA/CA junction, typically 50 mV. RA and CA are then chosen from standard value components to achieve the
above product. Typically CA is 3000 pF to 5000 pF and RA is 10 kΩ to 300 kΩ. CB is then chosen large
compared to CA, typically 0.1 µF. R1 and R2 should each be towards the upper end of the 2 kΩ to 10 kΩ range.
LM34919C
L1
SW
RA
CA
FB
CB
R1
V_OUT
C2
R2
Figure 13. Minimum Output Ripple Using Ripple Injection
c) Alternate minimum ripple configuration: The circuit in Figure 14 is the same as that in Figure 9, except the
output voltage is taken from the junction of R3 and C2. The ripple at V_OUT is determined by the inductor ripple
current and C2’s characteristics. R3 slightly degrades the load regulation because the feedback resistors are not
directly connected to V_OUT. This circuit may be suitable if the load current is fairly constant.
L1
SW
LM34919C
FB
R1
R3
V_OUT
R2
C2
Figure 14. Alternate Minimum Output Ripple Configuration
Minimum Load Current
20
Submit Documentation Feedback
Product Folder Links: LM34919C-Q1
Copyright © 2013, Texas Instruments Incorporated