t
ON
=
V
IN
- 1.35V
1.16 x 10
-10
x
(R1 + 1.4 k
:
)
+ 100 ns
Theory of Operation
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2
Theory of Operation
Refer to the evaluation board schematic in
Figure 2
, which contains a simplified block diagram of the
LM34917A. When the circuit is in regulation, the buck switch is on each cycle for a time determined by R1
and VIN according to the equation:
(1)
The on-time of this evaluation board ranges from
≊
510 ns at VIN = 8V, to
≊
186 ns at VIN = 33V. The on-
time varies inversely with VIN to maintain a nearly constant switching frequency. At the end of each on-
time the Minimum Off-Timer ensures the buck switch is off for at least 90 ns. In normal operation, the off-
time is much longer. During the off-time, the load current is supplied by the output capacitor (C7, C8).
When the output voltage falls sufficiently that the voltage at FB is below 2.5V, the regulation comparator
initiates a new on-time period. For stable, fixed frequency operation, a minimum of 25 mV of ripple is
required at FB to switch the regulation comparator. The current limit threshold, which varies with Vin, is
≊
1.4A at Vin = 8V, and
≊
1.2A at Vin = 33V. Refer to the LM34917A data sheet for a more detailed block
diagram, and a complete description of the various functional blocks.
3
Board Layout and Probing
Figure 1
shows the placement of the circuit components. The following should be kept in mind when the
board is powered:
•
When operating at high input voltage and high load current, forced air flow may be necessary.
•
The LM34917A, and diode D1 may be hot to the touch when operating at high input voltage and high
load current.
•
Use CAUTION when probing the circuit at high input voltages to prevent injury, as well as possible
damage to the circuit.
•
At maximum load current (1A), the wire size and length used to connect the load becomes important.
Ensure there is not a significant drop in the wires between this evaluation board and the load.
4
Board Connection/Start-Up
The input connections are made to the J1 connector. The load is connected to the J2 (OUT) and J3
(GND) terminals. Ensure the wires are adequately sized for the intended load current. Before start-up, a
voltmeter should be connected to the input terminals and to the output terminals. The load current should
be monitored with an ammeter or a current probe. It is recommended that the input voltage be increased
gradually to 8V, at which time the output voltage should be 5V. If the output voltage is correct with 8V at
VIN, then increase the input voltage as desired and proceed with evaluating the circuit. Do not exceed
50V at VIN.
5
Output Ripple Control
The LM34917A requires a minimum of 25 mVp-p ripple at the FB pin, in phase with the switching
waveform at the SW pin, for proper operation. The required ripple can be supplied from ripple at V
OUT
,
through the feedback resistors, as described in options B and C below, or the ripple can be generated
separately (using R5, C9, C10) keeping the ripple at V
OUT
to a minimum as described in option A.
2
AN-1601 LM34917A Evaluation Board
SNOA484D – June 2007 – Revised April 2013
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