LED7708 STMicroelectronics, LED7708 Datasheet - Page 27

• Manufacturer Part Number: LED7708
• Description: LED Lighting Drivers 16 Ch 85mA LED 3.6 to 36V 250kHz
• Manufacturer: STMicroelectronics

Specifications of LED7708

• Rohs: yes

LED7708
8.4
Detailed information about the MOSFET gate driver can be found in
characteristics.
The current flowing through the power MOSFET is sensed during the on-phase by
measuring the voltage drop across an external sensing resistor (connected between the
source of the MOSFET and PGND). The overcurrent condition is detected by comparing the
sensed voltage with an internal threshold. This threshold can be set in the 100 mV - 300 mV
range according to the resistor connected between the CLIM pin and ground. The sensing
resistors are often available in few values: once a suitable one has been selected, the
resistor at the CLIM pin is calculated according to the following equation:
Equation 9
where K
The maximum allowed peak current depends on the external MOSFET capability.
The actual maximum peak current is given by the above equations as long as duty cycle is
below 35% and decreases as duty cycle increases beyond 35%, depending on the slope
compensation setting. If an overcurrent (OC) condition occurs, the boost controller turns-off
the external MOSFET and skips subsequent switching cycles until the current is above the
programmed level.
In the case of a light load condition, the COMP pin voltage may fall below 0.8 V (typ.). In this
case the boost controller stops switching until the error amplifier forces the COMP voltage to
rise. The above voltage at the COMP pin corresponds to about 25% of full current limit.
In a boost converter, the RMS current through the internal MOSFET depends on both the
input and output voltages, according to
Equation 10
Equation 11
The current limitation works by clamping the COMP pin voltage proportionally to R
peak inductor current is limited to the above threshold decreased by the slope
compensation contribution.
Current generators section
The LED-driving section consists of 16 current generators connected to the internal control
logic. Each channel can sustain up to 40 V and sink up to 85 mA. The brightness of the
LEDs connected to the LED7708 is controlled by switching the current between a maximum
value (on-phase) and a minimum one (off-phase) at a given dimming frequency (PWM
control).
C1
= 1.63 x 10
I
MOS
-5
,
rms
V and K
=
I
OUT
R
Doc ID 022698 Rev 1
CLIM
C2
⎛
⎜
⎜ ⎜
⎝
I
(
MOS rms
1
= 3.1 x 10
=
−
D
(
-------------------------------------------------------------------- -
D
R
,
Equation 10
)
SNS
2
+
=
12
⋅
1
-2
--------------------- - D
F
V
I
SW
MOS OCP
⎛
⎜
⎜
⎝
IN
A.
K
I
OUT
C1
⋅
⋅
,
D
V
(DCM) and
Recommended operating conditions
L
OUT
⋅
f
SW
--- -
3
+
K
⋅
L
C2
⎞
⎟
⎟
⎠
2
)
(
D
11
(
1
−
(CCM).
D
Section 5: Electrical
)
)
3
⎞
⎟
⎟ ⎟
⎠
BILIM
. The
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