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LT3954IUHE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT3954
10
3954fa
For more information www.linear.com/3954
BLOCK DIAGRAM
+
–
+
–
+
–
–
+
–
+
×1/4
A6
+
+
–
FREQ
PROG
1V
1V
CLAMP
100mV
CTRL
V
REF
EN/UVLO
25mV
PWMINT
180µA
2.2µA
CTRL
BUFFER
CURRENT MODE
COMPARATOR
DRIVER
I
SENSE
A4
+
–
48mV
M1
g
m
A5
OVFB
COMPARATOR
1.25V
FB
PWMOUT
PWM
PWMINT
1.25V
V
IN
INTV
CC
V
C
+
–
+
–
A2
R Q
S
R Q
S
RAMP
GENERATOR
I
DIM/SS
DETECT
100kHz TO 1MHz
OSCILLATOR
+
–
+
–
A8
7.85V
LDO
SW
PGND
3954 BD
VMODE
GND
GNDK
1.2V
FB
ISN
ISP
+
–
1.22V
+
–
2V
1.3V
FB
0.8V + F3(I
DIM/SS
)
0.8V
RT
DIM/SS SYNC
SHDN
CV EAMP
CC EAMP
A7
10µA AT
FB = 1.25V
12µA
FAULT
LOGIC
T > 165°C
ISP > ISN + 0.6V
FAULT
10µA
BANDGAP
REFERENCE
OPENLED
LOGIC
–
+
g
m
A1
A3
ISN
ISP
F1(I
DIM/SS
)
F2(I
DIM/SS
)
10µA AT
A1
+
= A1
–
R
SENSE
+
–
+
–
+
–
1.5mA
FAULT
LT3954
11
3954fa
For more information www.linear.com/3954
The LT3954 is a constant-frequency, current mode con-
verter with a low side N-channel MOSFET switch. The
switch and PWMOUT pin drivers
, and other chip loads,
are powered from INTV
CC
, which is an internally regulated
supply. In the discussion that follows it will be helpful to
refer to the Block Diagram of the IC. In normal operation
with the PWM pin low, the switch is turned off and the
PWMOUT pin is driven to GND, the V
C
pin is high imped-
ance to store the previous switching state on the external
compensation capacitor
, and the ISP and ISN pin bias
currents are reduced to leakage levels. When the PWM pin
transitions high, the PWMOUT pin transitions high after a
short delay. At the same time, the internal oscillator wakes
up and generates a pulse to set the PWM latch, turning on
the internal power MOSFET switch. A voltage input propor
-
tional to the switch current, sensed by an internal current
sense resistor is added to a stabilizing slope compensation
ramp and the resulting switch current sense signal is fed
into
the negative terminal of the PWM comparator. The
current in the external inductor increases steadily during
the time the switch is on. When the switch current sense
voltage exceeds the output of the error amplifier, labeled
V
C
, the latch is reset and the switch is turned off. During
the switch-off phase, the inductor current decreases. At the
completion of each oscillator cycle, internal signals such
as slope compensation return to their starting points and
a new cycle begins with the set pulse from the oscillator.
Through this repetitive action, the PWM control algorithm
establishes a switch duty cycle to regulate a current or
voltage in the load. The V
C
signal is integrated over many
switching cycles and is an amplified version of the differ-
ence between the LED current sense voltage, measured
between ISP and ISN
,
and the target difference voltage
set by the CTRL pin. In this manner, the error amplifier
sets the correct peak switch current level to keep the
LED current in regulation. If the error amplifier output
increases, more current is demanded in the switch; if it
decreases, less current is demanded. The switch current
is monitored during the on-phase and is not allowed to
exceed the current limit threshold of 6.0A (typical). If the
SW pin exceeds the current limit threshold, the SR latch is
reset regardless of the output state of the PWM compara
-
tor. The difference between ISP and ISN is monitored to
determine if the output is in a short-cir
cuit condition
. If
the difference between ISP and ISN is greater than 600mV
(typical), the SR latch will be reset regardless of the PWM
comparator. The DIM/SS pin will be pulled down and the
PWMOUT pin forced low and the SW pin turned off for
at least 4µs. These functions are intended to protect the
power switch as well as various external components in
the power path of the DC/DC converter.
In voltage feedback mode, the operation is similar to that
described above, except the voltage at the VC pin is set by
the amplified difference of the internal reference of 1.25V
and the FB pin. If FB is lower than the reference voltage,
the switch current will increase; if FB is higher than the
reference voltage, the switch demand current will decrease.
The LED current sense feedback interacts with the FB
voltage feedback so that FB will not exceed the internal
reference and the voltage between ISP and ISN will not
exceed the threshold set by the CTRL pin. For accurate
current or voltage regulation, it is necessary to be sure that
under normal operating conditions the appropriate loop is
dominant. To deactivate the voltage loop entirely, FB can
be connected to GND. To deactivate the LED current loop
entirely, the ISP and ISN should be tied together and the
CTRL input tied to V
REF
.
Two LED specific functions featured on the LT3954 are
controlled by the voltage feedback pin. First, when the
FB pin exceeds a voltage 50mV lower (–4%) than the FB
regulation voltage, and the difference voltage between
ISP and ISN is below 25mV (typical), the pull-down driver
on the VMODE pin is activated. This function provides a
status indicator that the load may be disconnected and
the constant-voltage feedback loop is taking control of the
switching regulator. The VMODE pin de-asserts only when
PWM is high and FB drops below the voltage threshold. FB
overvoltage is the second protective function. When the
FB pin exceeds the FB regulation voltage by 60mV (plus
5% typical), the PWMOUT pin is driven low, ignoring the
state of the PWM input. In the case where the PWMOUT
pin drives a disconnect NFET, this action isolates the
LED load from GND, preventing excessive current from
damaging the LEDs.
OPERATION
LT3954
12
3954fa
For more information www.linear.com/3954
APPLICATIONS INFORMATION
INTV
CC
Regulator Bypassing and Operation
The INTV
CC
pin requires a capacitor for stable operation
and to store the charge for the large internal MOSFET gate
switching currents. Choose a 10V rated low ESR, X7R
ceramic capacitor for best performance. A 1μF capacitor
will be adequate for many applications. Place the capacitor
close to the IC to minimize the trace length to the INTV
CC
pin and also to the IC ground.
An internal current limit on the INTV
CC
output protects the
LT3954 from excessive on-chip power dissipation. The
INTV
CC
pin has its own undervoltage disable set to 4.1V
(typical) to protect the internal MOSFET from excessive
power dissipation caused by not being fully enhanced.
If the INTV
CC
pin drops below the UVLO threshold, the
PWMOUT pin will be forced to 0V, the power switch will
be turned off and the soft-start pin will be reset.
If the input voltage, V
IN
, will not exceed 8.1V, then the
INTV
CC
pin could be connected to the input supply. Be
aware that a small current (less than 13μA) will load the
INTV
CC
in shutdown. This action allows the LT3954 to
operate from V
IN
as low as 4.5V. If V
IN
is normally above,
but occasionally drops below the INTV
CC
regulation voltage,
then the minimum operating V
IN
will be close to 5V. This
value is determined by the dropout voltage of the linear
regulator and the INTV
CC
undervoltage lockout threshold
mentioned above.
Programming the Turn-On and Turn-Off Thresholds
with the EN/UVLO Pin
The power supply undervoltage lockout (UVLO) value
can be accurately set by the resistor divider to the
EN/UVLO pin. A small 2.2μA pull-down current is active when
EN/UVLO is below the threshold. The purpose of this cur-
rent is to allow the user to program the rising hysteresis.
The following equations should be used to determine the
value of the resistors
:
V
IN,FALLING
=1.22•
R1+R2
R2
V
IN,RISING
= 2.2µA•R1 + V
IN,FALLING
EN/UVLO
LT3954
V
IN
R2
3954 F01
R1
Figure 1. Resistor Connection to Set
V
IN
Undervoltage Shutdown Threshold
LED Current Programming
The LED current is programmed by placing an appropriate
value current sense resistor, R
LED
, in series with the LED
string. The voltage drop across R
LED
is (Kelvin) sensed by
the ISP and ISN pins. A half watt resistor is usually a good
choice. To give the best accuracy, sensing of the current
should be done at the top of the LED string. If this option is
not available then the current may be sensed at the bottom
of the string, or in the source of the PWM disconnect NFET
driven by the PWMOUT signal. Input bias currents for the
ISP and ISN inputs are shown in the typical performance
characteristics and should be considered when placing a
resistor in series with the ISP or ISN pins.
The CTRL pin should be tied to a voltage higher than 1.2V
to get the full-scale 250mV (typical) threshold across the
sense resistor. The CTRL pin can also be used to dim the
LED current to zero, although relative accuracy decreases
with the decreasing voltage sense threshold. When the
CTRL pin voltage is less than 1V, the LED current is:
I
LED
=
V
CTRL
−100mV
R
LED
• 4
When the CTRL pin voltage is between 1V and 1.2V the LED
current varies with CTRL, but departs from the previous
equation by an increasing amount as the CTRL voltage
increases. Ultimately, the LED current no longer varies for
CTRL ≥ 1.2V. At CTRL = 1.1V, the value of I
LED
is ~98% of
the equation’s estimate. Some values are listed in Table 1.
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LT3954IUHE#PBF

Mfr. #:
Buy LT3954IUHE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers 40Vin, 5A LED Driver with Internal PWM Driver
Lifecycle:
New from this manufacturer.
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