LM34914EVAL National Semiconductor, LM34914EVAL Datasheet - Page 10

• 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: -

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at high line and load. To further reduce the inductor’s peak
current, the next cycle’s on-time is reduced by approximately
50% if the voltage at FB is below its threshold when the in-
ductor current reduces to the current limit threshold (V
low due to current limiting).
Figure 3 illustrates the inductor current waveform during nor-
mal operation and in current limit. During the first “Normal
Operation” the load current is I
waveform. As the load resistance is reduced, the inductor
current increases until it exceeds the current limit threshold.
During the “Current Limited” portion of Figure 3, the current
limit threshold lowers since the high load current causes
V
duced by approximately 50%, resulting in lower ripple ampli-
N - Channel Buck Switch and Driver
The LM34914 integrates an N-Channel buck switch and as-
sociated floating high voltage gate driver. The gate driver
circuit works in conjunction with an external bootstrap capac-
itor and an internal high voltage diode. A 0.022 µF capacitor
(C4) connected between BST and SW provides the voltage
to the driver during the on-time. During each off-time, the SW
pin is at approximately -1V, and C4 is recharged for the next
on-time from V
off-time ensures a minimum time each cycle to recharge the
bootstrap capacitor.
Softstart
The softstart feature allows the converter to gradually reach
a steady state operating point, thereby reducing start-up
stresses and current surges. Upon turn-on, after V
the under-voltage threshold, an internal 12.5 µA current
source charges up the external capacitor at the SS pin to 2.5V
(t
verting input of the regulation comparator) ramps up the
output voltage in a controlled manner.
2
OUT
in Figure 1). The ramping voltage at SS (and the non-in-
(and the voltage at FB) to reduce. The on-time is re-
CC
through the internal diode. The minimum
FIGURE 3. Inductor Current - Normal and Current Limit Operation
OUT1
, the average of the ripple
CC
reaches
OUT
is
10
tude for the inductor’s current. During this time the LM34914
is in a constant current mode, with an average load current
equal to the current limit threshold + ΔI/2 (I
eration resumes when the load current is reduced to I
allowing V
return to their normal values. Note that in the second period
of “Normal Operation”, even though the inductor’s peak cur-
rent exceeds the current limit threshold during part of each
cycle, the circuit is not in current limit since the current falls
below the threshold before the feedback voltage reduces to
its threshold.
The peak current allowed through the buck switch, and the
ISEN pin, is 2A, and the maximum allowed average current
is 1.5A.
An internal switch grounds the SS pin if V
der-voltage lockout threshold, or if the RON/SD pin is ground-
ed.
Thermal Shutdown
The LM34914 should be operated so the junction temperature
does not exceed 125°C. If the junction temperature increases
above that, an internal Thermal Shutdown circuit activates
(typically) at 175°C, taking the controller to a low power reset
state by disabling the buck switch and the on-timer. This fea-
ture helps prevent catastrophic failures from accidental de-
vice overheating. When the junction temperature reduces
below 155°C (typical hysteresis = 20°C), normal operation
resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select the external
components (see the Block Diagram). First determine the fol-
lowing operating parameters:
- Output voltage (V
- Minimum and maximum input voltage (V
OUT
, the current limit threshold, and the on-time to
OUT
)
CC
IN(min)
OUT2
is below the un-
). Normal op-
and V
20197317
IN(max)
OUT3
)
,