LM34914EVAL National Semiconductor, LM34914EVAL Datasheet - Page 9

• Manufacturer Part Number: LM34914EVAL
• Description: BOARD EVALUATION LM34914
• Manufacturer: National Semiconductor

Specifications of LM34914EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 1A
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 275kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34914
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

and line variations. The minimum load current for continuous
conduction mode is one-half the inductor’s ripple current am-
plitude. The approximate operating frequency is calculated as
follows:
The buck switch duty cycle is equal to:
In discontinuous conduction mode, where the inductor’s cur-
rent reaches zero during the off-time forcing a longer-than-
normal off-time, the operating frequency is lower than in
continuous conduction mode, and varies with load current.
Conversion efficiency is maintained at light loads since the
switching losses decrease with the reduction in load and fre-
quency. The approximate discontinuous operating frequency
can be calculated as follows:
where R
The output voltage is set by the two feedback resistors (R1,
R2 in the Block Diagram). The regulated output voltage is
calculated as follows:
Output voltage regulation is based on supplying ripple voltage
to the feedback input (FB pin), normally obtained from the
output voltage ripple through the feedback resistors. The
LM34914 requires a minimum of 25 mVp-p of ripple voltage
at the FB pin, requiring the ripple voltage at V
by the gain factor of the feedback resistor ratio. The output
ripple voltage is created by the inductor’s ripple current pass-
ing through R3 which is in series with the output capacitor.
For applications where reduced ripple is required at V
the Applications Information section.
If the voltage at FB rises above 2.9V, due to a transient at
V
normal ripple at V
immediately shuts off the internal buck switch. The next on-
time starts when the voltage FB falls below 2.5V and the
inductor current falls below the current limit threshold.
OUT
or excessive inductor current which creates higher than
L
= the load resistance, and L1 is the circuit’s inductor.
V
OUT
OUT
, the internal over-voltage comparator
= 2.5 x (R1 + R2) / R2
OUT
be higher
OUT
, see
(1)
(2)
(3)
9
ON-Time Timer
The on-time for the LM34914 is determined by the R
tor and the input voltage (V
The inverse relationship with V
frequency as V
duction mode switching frequency (F
determined from the following:
Equations 1, 4 and 5 are valid only during normal operation -
i.e., the circuit is not in current limit. When the LM34914
operates in current limit, the on-time is reduced by approxi-
mately50%. This feature reduces the peak inductor current
which may be excessively high if the load current and the input
voltage are simultaneously high. This feature operates on a
cycle-by-cycle basis until the load current is reduced and the
output voltage resumes its normal regulated value.
Shutdown
The LM34914 can be remotely shut down by taking the RON/
SD pin below 0.8V. See Figure 2. In this mode the SS pin is
internally grounded, the on-timer is disabled, and bias cur-
rents are reduced. Releasing the RON/SD pin allows the
circuit to resume operation. The voltage at the RON/SD pin is
normally between 1.5V and 3.0V, depending on V
R
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the recirculating current flowing out of the ISEN pin.
Referring to the Block Diagram, during the off-time the induc-
tor current flows through the load, into SGND, through the
internal sense resistor, out of ISEN and through D1 to the
inductor. If that current exceeds the current limit threshold the
current limit comparator output delays the start of the next on-
time period. The next on-time starts when the current out of
ISEN is below the threshold and the voltage at FB falls below
2.5V. The operating frequency is typically lower due to longer-
than-normal off-times.
The valley current limit threshold is a function of the input
voltage (V
in the graph “Valley Current Limit Threshold vs. V
V
IN
ON
”. This feature reduces the inductor current’s peak value
resistor.
FIGURE 2. Shutdown Implementation
IN
) and the output voltage sensed at FB, as shown
IN
is varied. To set a specific continuous con-
IN
), calculated from:
IN
results in a nearly constant
S
), the R
ON
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20197316
resistor is
IN
ON
and the
FB
resis-
and
(4)
(5)