LM34919EVAL National Semiconductor, LM34919EVAL Datasheet - Page 10

• Manufacturer Part Number: LM34919EVAL
• Description: BOARD EVAL FOR LM34919 SW REG
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM34919EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 5V
• Current - Output: 600mA
• Voltage - Input: 8 ~ 40V
• Regulator Topology: Buck
• Frequency - Switching: 800kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM34919
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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Regulation Comparator
The feedback voltage at FB is compared to the voltage at the
Softstart pin (2.5V). In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at
FB falls below 2.5V. The buck switch stays on for the pro-
grammed on-time, causing the FB voltage to rise above 2.5V.
After the on-time period, the buck switch stays off until the FB
voltage falls below 2.5V. Input bias current at the FB pin is
less than 100 nA over temperature.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.9V reference.
If the voltage at FB rises above 2.9V the on-time pulse is im-
mediately terminated. This condition can occur if the input
voltage or the output load changes suddenly, or if the inductor
(L1) saturates. The buck switch remains off until the voltage
at FB falls below 2.5V.
ON-Time Timer, and Shutdown
The on-time is determined by the R
voltage (V
The inverse relationship with V
frequency as V
duction mode switching frequency (F
determined from the following:
In high frequency applicatons the minimum value for t
limited by the maximum duty cycle required for regulation and
the minimum off-time of (155 ns, ±15%). The minimum off-
time limits the maximum duty cycle achievable with a low
voltage at V
limited to
The LM34919 can be remotely shut down by taking the RON/
SD pin below 0.8V. See Figure 3. 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 normal
operation to resume. The voltage at the RON/SD pin is be-
tween 1.4V and 4.0V, depending on V
≊
IN
FIGURE 3. Shutdown Implementation
), and is calculated from:
IN
120 ns.
. At high values of VIN, the minimum on-time is
IN
is varied. To set a specific continuous con-
IN
results in a nearly constant
ON
IN
S
resistor and the input
), the R
and the R
ON
30004413
ON
resistor is
resistor.
ON
(4)
(5)
is
10
Current Limit
Current limit detection occurs during the off-time by monitor-
ing the recirculating current through the free-wheeling diode
(D1). Referring to the Block Diagram, when the buck switch
is turned off the inductor current flows through the load, into
SGND, through the sense resistor, out of ISEN and through
D1. If that current exceeds 0.64A the current limit comparator
output switches to delay the start of the next on-time period.
The next on-time starts when the current out of ISEN is below
0.64A and the voltage at FB is below 2.5V. If the overload
condition persists causing the inductor current to exceed
0.64A during each on-time, that is detected at the beginning
of each off-time. The operating frequency is lower due to
longer-than-normal off-times.
Figure 4 illustrates the inductor current waveform. During nor-
mal operation the load current is Io, the average of the ripple
waveform. When the load resistance decreases the current
ratchets up until the lower peak reaches 0.64A. During the
Current Limited portion of Figure 4, the current ramps down
to 0.64A during each off-time, initiating the next on-time (as-
suming the voltage at FB is <2.5V). During each on-time the
current ramps up an amount equal to:
During this time the LM34919 is in a constant current mode,
with an average load current (I
Generally, in applications where the switching frequency is
higher than
higher dl/dt of the inductor's ripple current results in an effec-
tively lower valley current limit threshold due to the response
time of the current limit detection circuit. However, since the
small value inductor results in a relatively high ripple current
amplitude (ΔI in Figure 4), the load current (I
limit is typically in excess of 640 mA.
≊
300 kHz and uses a small value inductor, the
ΔI = (V
IN
- V
OUT
OCL
) x t
) equal to 0.64A + ΔI/2.
ON
/ L1
OCL
) at current