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LM3500 Datasheet(PDF) 13 Page - Texas Instruments

Part No. LM3500
Description  Synchronous Step-up DC/DC Converter for White LED Applications
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM3500 Datasheet(HTML) 13 Page - Texas Instruments

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+
-
+
-
osc
EAMP
LOGIC
Reset
DriveN
DriveP
+
-
Reset
LIGHT LOAD
COMP
REF
Reset Reset
SET Reset
+
-
Reset
+
-
OVP
REF
PWM
COMP
OVP
COMP
C3
B3
A3
A1
B1
CIN
L
VIN
FB
VIN
COUT
x
LED
R
FB
NC
SHDN
AGND
GND
VSW
VOUT
+
-
UVP
REF
UVP
COMP
Reset
Dlimit
Duty Limit
Comp
Current
Sense
THERMAL
SHUTDOWN
SHUTDOWN
COMP
Body Diode
Control
C2
C1
A2
0.5V
x
x
x
x
LM3500
www.ti.com
SNVS231F – MAY 2004 – REVISED APRIL 2007
Operation
Figure 2. LM3500 Block Diagram
The LM3500 utilizes a synchronous Current Mode PWM control scheme to regulate the feedback voltage over
almost all load conditions. The DC/DC controller acts as a controlled current source ideal for white LED
applications. The LM3500 is internally compensated preventing the use of any external compensation
components providing a compact overall solution. The operation can best be understood referring to the block
diagram in Figure 2. At the start of each cycle, the oscillator sets the driver logic and turns on the NMOS power
device conducting current through the inductor and turns off the PMOS power device isolating the output from
the VSW pin. The LED current is supplied by the output capacitor when the NMOS power device is active. During
this cycle, the output voltage of the EAMP controls the current through the inductor. This voltage will increase for
larger loads and decrease for smaller loads limiting the peak current in the inductor minimizing EMI radiation.
The EAMP voltage is compared with a voltage ramp and the sensed switch voltage. Once this voltage reaches
the EAMP output voltage, the PWM COMP will then reset the logic turning off the NMOS power device and
turning on the PMOS power device. The inductor current then flows through the PMOS power device to the white
LED load and output capacitor. The inductor current recharges the output capacitor and supplies the current for
the white LED branches. The oscillator then sets the driver logic again repeating the process. The Duty Limit
Comp is always operational preventing the NMOS power switch from being on more than one cycle and
conducting large amounts of current.
The LM3500 has dedicated protection circuitry active during normal operation to protect the IC and the external
components. The Thermal Shutdown circuitry turns off both the NMOS and PMOS power devices when the die
temperature reaches excessive levels. The LM3500 has a UVP Comp that disables both the NMOS and PMOS
power devices when battery voltages are too low preventing an on state of the power devices which could
conduct large amounts of current. The OVP Comp prevents the output voltage from increasing beyond
15.5V(LM3500-16) and 20.5V(LM3500-21) when the primary white LED network is removed or if there is an LED
failure, allowing the use of small (16V for LM3500-16 and 25V for LM3500-21) ceramic capacitors at the output.
This comparator has hysteresis that will regulate the output voltage between 15.5V and 14.6V typically for the
LM3500-16, and between 20.5V and 19.5V for the LM3500-21. The LM3500 features a shutdown mode that
reduces the supply current to 0.1uA and isolates the input and output of the converter.
Copyright © 2004–2007, Texas Instruments Incorporated
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