Datasheet

t
on
=
V
IN
- 0.5V
1.25 x 10
-10
x (R1 + 500:)
+ 30 ns
Theory of Operation www.ti.com
2 Theory of Operation
Figure 6 shows the evaluation board schematic. When the circuit is in regulation, the buck switch is on
each cycle for a time determined by R1 and VIN according to Equation 1:
(1)
The on-time of this evaluation board ranges from ≊4.38 µs at VIN = 6V, to ≊351 ns at VIN = 75V. 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 260 ns. In normal operation, the off-
time is much longer. During the off-time, the load current is supplied by the output capacitor (C2). 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. For a more detailed block diagram, and a complete description of
the various functional blocks, see the LM34923 80-V 600-mA Constant On-Time Buck Switching
Regulator Data Sheet (SNVS695).
3 Board Layout and Probing
The pictorial in 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 LM34923 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, 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 6V, at which time the output voltage should be 5V. If the output voltage is correct with 6V at
VIN, then increase the input voltage as desired and proceed with evaluating the circuit. DO NOT EXCEED
75V AT VIN.
5 Output Ripple Control
The LM34923 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 Section 5.1. Section 5.2 and Section 5.3 provide lower output ripple
with one or two additional components.
5.1 Option A) Lowest Cost Configuration
In this configuration R7 is installed in series with the output capacitance (C2). Since ≥25 mVp-p are
required at the FB pin, R7 must be chosen to generate ≥50 mVp-p at V
OUT
, knowing that the minimum
ripple current in this circuit is ≊51 mAp-p at minimum V
IN
. Using 1Ω for R7, the ripple at V
OUT
ranges from
≊51 mVp-p to ≊280 mVp-p over the input voltage range. If the application can accept this ripple level, this
is the most economical solution. The circuit is shown in Figure 2, see Figure 8. R8, C6, C7, and C8 are
not used in this configuration.
2
AN-2147 LM34923 Evaluation Board SNVA482A–May 2011–Revised April 2013
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