LM34917A Datasheet, PDF (14/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

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FIGURE 9. Alternate Minimum Output Ripple
Configuration
Minimum Load Current
The LM34917A 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 appli-
cation, 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 LM34917A 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 com-
pact 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 LM34917A can be approxi-
mated 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 ap-
proximately:
P_D1 = Iout x V_F x (1-D)
where Iout is the load current, V_F is the diode’s forward volt-
age 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
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 LM34917A will produce excessive
junction temperatures during normal operation, good use of
the PC board’s ground plane can help to dissipate heat. Ad-
ditionally 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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