LM3500TL-16EV National Semiconductor, LM3500TL-16EV Datasheet - Page 13

• Manufacturer Part Number: LM3500TL-16EV
• Description: BOARD EVALUATION LM3500TL-16
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3500TL-16EV

• Current - Output / Channel: 20mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 14V
• Features: Shutdown
• Voltage - Input: 2.7 ~ 5.5 V
• Utilized Ic / Part: LM3500
• Core Chip: LM3500
• Topology: Buck
• No. Of Outputs: 1
• Output Voltage: 16V
• Input Voltage: 2.7V To 7V
• Kit Contents: Board
• Development Tool Type: Hardware - Eval/Demo Board
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Application Information
ADJUSTING LED CURRENT
The White LED current is set using the following equation:
The LED current can be controlled using a PWM signal on the
SHDN pin with frequencies in the range of 100Hz (greater
than visible frequency spectrum) to 1kHz. For controlling LED
currents down to the µA levels, it is best to use a PWM signal
frequency between 200-500Hz. The LM3500 LED current can
be controlled with PWM signal frequencies above 1kHz but
the controllable current decreases with higher frequency. The
maximum LED current would be achieved using the equation
above with 100% duty cycle, ie. the SHDN pin always high.
LED-DRIVE CAPABILITY
The maximum number of LEDs that can be driven by the
LM3500 is limited by the output voltage capability of the
LM3500. When using the LM3500 in the typical application
configuration, with LEDs stacked in series between the
V
placed in series (N
forward voltage (V
pin (V
protection level of the chosen LM3500 option (LM3500-16:
OVP
function properly, the following inequality must be met:
When inserting a value for maximim LED V
voltage variation over the operating temperature range
should be considered. The table below provides maximum
LED voltage numbers for the LM3500-16 and LM3500-21 in
the typical application circuit configuration (with 3, 4, 5, 6, or
7 LEDs placed in series between the V
For the LM3500 to operate properly, the output voltage must
be kept above the input voltage during operation. For most
applications, this requires a minimum of 2 LEDs (total of 6V
or more) between the FB and V
OUTPUT OVERVOLTAGE PROTECTION
The LM3500 contains dedicated circuitry for monitoring the
output voltage. In the event that the primary LED network is
disconnected from the LM3500-16, the output voltage will in-
crease and be limited to 15.5V (typ.). There is a 900mV
hysteresis associated with this circuitry which will cause the
output to fluctuate between 15.5V and 14.6V (typ.) if the pri-
mary network is disconnected. In the event that the network
is reconnected regulation will begin at the appropriate output
voltage. The 15.5V limit allows the use of 16V 1µF ceramic
output capacitors creating an overall small solution for white
LED applications.
In the event that the primary LED network is disconnected
from the LM3500-21, the output voltage will increase and be
OUT
(N
MIN
(in series)
# of LEDs
and FB pins, the maximum number of LEDs that can be
FB-MAX
MAX
= 15V; LM3500-21: OVP
3
4
5
6
7
× V
= 0.53V), and the minimum output over-voltage
F-MAX
F-MAX
MAX
) + 0.53V
) is dependent on the maximum LED
), the voltage of the LM3500 feedback
LM3500-16
4.82V
3.61V
2.89V
≤
X
X
Maximum LED V
OUT
OVP
MIN
pins.
= 20V). For the circuit to
MIN
OUT
and FB pins).
LM3500-21
F
, LED forward
6.49V
4.86V
3.89V
3.24V
2.78V
F
13
limited to 20.5V (typ.). There is a 1V hysteresis associated
with this circuitry which will cause the output to fluctuate be-
tween 20.5V and 19.5V (typ.) if the primary network is dis-
connected. In the event that the network is reconnected
regulation will begin at the appropriate output voltage. The
20.5V limit allows the use of 25V 1µF ceramic output capac-
itors.
RELIABILITY AND THERMAL SHUTDOWN
The maximum continuous pin current for the 8 pin thin micro
SMD package is 535mA. When driving the device near its
power output limits the V
than 535mA (see INDUCTOR SELECTION section for aver-
age switch current). To preserve the long term reliability of the
device the average switch current should not exceed 535mA.
The LM3500 has an internal thermal shutdown function to
protect the die from excessive temperatures. The thermal
shutdown trip point is typically 150°C. There is a hysteresis of
typically 35°C so the die temperature must decrease to ap-
proximately 115°C before the LM3500 will return to normal
operation.
INDUCTOR SELECTION
The inductor used with the LM3500 must have a saturation
current greater than the cycle by cycle peak inductor current
(see Typical Peak Inductor Currents table below). Choosing
inductors with low DCR decreases power losses and increas-
es efficiency.
The minimum inductor value required for the LM3500-16 can
be calculated using the following equation:
The minimum inductor value required for the LM3500-21 can
be calculated using the following equation:
For both equations above, L is in µH, V
of the chip in Volts, R
power switch found in the Typical Performance Characteris-
tics section in ohms and D is the duty cycle of the switching
regulator. The above equation is only valid for D greater than
or equal to 0.5. For applications where the minimum duty cy-
cle is less than 0.5, a 22µH inductor is the typical recommen-
dation for use with most applications. Bench-level verification
of circuit performance is required in these special cases, how-
ever. The duty cycle, D, is given by the following equation:
where V
OUT
is the voltage at pin C1.
DSON
SW
is the ON resistance of the NMOS
pin can see a higher DC current
IN
is the input supply
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