Datasheet

FB
SW
L1
R1
LM34919C
R2
R3
C2
V
OUT
LM34919B
LM34919B-Q1
SNVS623A –MAY 2010–REVISED FEBRUARY 2013 www.ti.com
Figure 34. Alternate Minimum Output Ripple Configuration
Minimum Load Current
The LM34919B requires a minimum load current of 1 mA. If the load current falls below that level, the bootstrap
capacitor (C4) may discharge during the long off-time, and the circuit will either shutdown, or cycle on and off at
a low frequency. If the load current is expected to drop below 1 mA in the application, R1 and R2 should be
chosen low enough in value so they provide the minimum required current at nominal V
OUT
.
PC BOARD LAYOUT
Refer to application note AN-1112 for PC board guidelines for the DSBGA package.
The LM34919B regulation, over-voltage, and current limit comparators are very fast, and respond to short
duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be
as neat and compact as possible, and all of the components must be as close as possible to their associated
pins. The two major current loops have currents which switch very fast, and so the loops should be as small as
possible to minimize conducted and radiated EMI. The first loop is that formed by C1, through the VIN to SW
pins, L1, C2, and back to C1.The second current loop is formed by D1, L1, C2 and the SGND and ISEN pins.
The power dissipation within the LM34919B can be approximated by determining the total conversion loss (P
IN
-
P
OUT
), and then subtracting the power losses in the free-wheeling diode and the inductor. The power loss in the
diode is approximately:
P
D1
= Iout x V
F
x (1-D) (20)
where Iout is the load current, V
F
is the diode’s forward voltage drop, and D is the on-time duty cycle. The power
loss in the inductor is approximately:
P
L1
= Iout
2
x R
L
x 1.1 (21)
where R
L
is the inductor’s DC resistance, and the 1.1 factor is an approximation for the AC losses. If it is
expected that the internal dissipation of the LM34919B will produce excessive junction temperatures during
normal operation, good use of the PC board’s ground plane can help to dissipate heat. Additionally the use of
wide PC board traces, where possible, can help conduct heat away from the IC. Judicious positioning of the PC
board within the end product, along with the use of any available air flow (forced or natural convection) can help
reduce the junction temperatures.
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