LM5009EVAL/NOPB National Semiconductor, LM5009EVAL/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM5009EVAL/NOPB
• Description: EVAL BOARD FOR LM5009
• Manufacturer: National Semiconductor

Specifications of LM5009EVAL/NOPB

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Current Limit
The LM5009 contains an intelligent current limit OFF timer. If
the current in the Buck switch exceeds 0.31A the present cy-
cle is immediately terminated, and a non-resetable OFF timer
is initiated. The length of off-time is controlled by an external
resistor (R
off-time is required, and the time is preset to 35µs. This con-
dition occurs when the output is shorted, and during the initial
part of start-up. This amount of time ensures safe short circuit
operation up to the maximum input voltage of 95V. In cases
of overload where the FB voltage is above zero volts (not a
short circuit) the current limit off-time will be less than 35µs.
Reducing the off-time during less severe overloads reduces
the amount of foldback, recovery time, and the start-up time.
The off-time is calculated from the following equation:
The current limit sensing circuit is blanked for the first 50-70ns
of each on-time so it is not falsely tripped by the current surge
which occurs at turn-on. The current surge is required by the
re-circulating diode (D1) for its turn-off recovery.
N - Channel Buck Switch and Driver
The LM5009 integrates an N-Channel buck switch and asso-
ciated floating high voltage gate driver. The gate driver circuit
works in conjunction with an external bootstrap capacitor and
an internal high voltage diode. A 0.022 µF ceramic capacitor
(C4) connected between the BST pin and SW pin provides
the voltage to the driver during the on-time.
During each off-time, the SW pin is at approximately -1V, and
the bootstrap capacitor charges from Vcc through the internal
diode. The minimum OFF timer ensures a minimum time each
cycle to recharge the bootstrap capacitor.
An external re-circulating diode (D1) carries the inductor cur-
rent after the internal buck switch turns off. This diode should
be of the Ultra-fast or Schottky type to minimize turn-on losses
and current over-shoot.
Thermal Protection
The LM5009 should be operated so the junction temperature
does not exceed 125°C during normal operation. An internal
Thermal Shutdown circuit is provided to protect the LM5009
in the event of a higher than normal junction temperature.
CL
) and the FB voltage. When FB = 0V, a maximum
FIGURE 4. Shutdown Implementation
(3)
9
When activated, typically at 165°C, the controller is forced into
a low power reset state, disabling the buck switch. This fea-
ture prevents catastrophic failures from accidental device
overheating. When the junction temperature reduces below
140°C (typical hysteresis = 25°C), the buck switch is enabled,
and normal operation is resumed.
Applications Information
SELECTION OF EXTERNAL COMPONENTS
A guide for determining the component values will be illus-
trated with a design example. Refer to
steps will configure the LM5009 for:
•
•
•
R1 and R2: From
since V
Standard values of 3.01 kΩ (R1) and 1.00 kΩ (R2) are cho-
sen. Other values could be used as long as the 3:1 ratio is
maintained. The selected values, however, provide a small
amount of output loading (2.5 mA) in the event the main load
is disconnected. This allows the circuit to maintain regulation
until the main load is reconnected.
F
quency, the choice of frequency is generally a compromise
since it affects the size of L1 and C2, and the switching losses.
The maximum allowed frequency, based on a minimum on-
time of 250 ns, is calculated from:
For this exercise, Fmax = 444 kHz. From equation 1, R
calculates to 180 kΩ. A standard value 237 kΩ resistor will be
used to allow for tolerances in equation 1, resulting in a nom-
inal frequency of 337 kHz.
L1: The main parameter affected by the inductor is the output
current ripple amplitude. The choice of inductor value there-
fore depends on both the minimum and maximum load cur-
rents, keeping in mind that the maximum ripple current occurs
at maximum Vin.
a) Minimum load current: To maintain continuous conduc-
tion at minimum Io (100 mA), the ripple amplitude (I
be less than 200 mA p-p so the lower peak of the waveform
does not reach zero. L1 is calculated using the following
equation:
s
Input voltage range (Vin): 12V to 90V
Output voltage (V
Load current (for continuous conduction mode): 100mA to
150mA
and R
FB
ON
= 2.5V, the ratio of R1 to R2 calculates as 3:1.
: Unless the application requires a specific fre-
F
20165807
MAX
Figure
= V
OUT1
OUT
1, V
): 10V
/ (V
OUT1
INMAX
= V
x 250 ns)
FB
Figure
x (R1 + R2) / R2, and
1. The following
www.national.com
OR
) must
ON