LM34919_14 Datasheet, PDF (15/21 Pages) Texas Instruments – Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator

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LM34919
L1
SW
FB
LM34919
SNOSAY2D – MAY 2007 – REVISED JUNE 2008
R1
R3
V_OUT
R2
C2
Figure 12. Alternate Minimum Output Ripple Configuration
Minimum Load Current
The LM34919 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 Micro SMD package.
The LM34919 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 LM34919 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 = Iout² 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 LM34919 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.
Copyright © 2007–2008, Texas Instruments Incorporated
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