LED7707 STMicroelectronics, LED7707 Datasheet - Page 16

• Manufacturer Part Number: LED7707
• Description: LED Drivers 6-ROW 85mA LED DRIV BOOST REG LCD PANEL
• Manufacturer: STMicroelectronics

Specifications of LED7707

• High Level Output Current: 85 mA
• Operating Supply Voltage: 4.5 V to 36 V
• Maximum Supply Current: 1 mA
• Maximum Power Dissipation: 2.3 W
• Maximum Operating Temperature: + 150 C
• Mounting Style: SMD/SMT
• Package / Case: VFQFPN-24
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Operation description
5.1.6
16/47
Figure 11. External sync waveforms
Slope compensation
The constant frequency, peak current-mode topology has the advantage of very easy loop
compensation with output ceramic caps (reduced cost and size of the application) and fast
transient response. In addition, the intrinsic peak-current measurement simplifies the
current limit protection, avoiding undesired saturation of the inductor.
On the other side, this topology has a drawback: there is an inherent open loop instability
when operating with a duty-ratio greater than 0.5. This phenomenon is known as “Sub-
Harmonic Instability” and can be avoided by adding an external ramp to the one coming
from the sensed current. This compensating technique, based on the additional ramp, is
called “slope compensation”. In
the small perturbation ΔI
and the system reverts to a stable situation.
The SLOPE pin allows to properly set the amount of slope compensation connecting a
simple resistor R
starts at 35% (typ.) of each switching period and its slope is given by the following equation:
Equation 5
Where K
To avoid sub-harmonic instability, the compensating slope should be at least half the slope
of the inductor current during the off-phase when the duty-cycle is greater than 50%. The
value of R
Slave SYNC pin voltage
Slave LX pin voltage
FSW pin voltage (ext. sync)
S
SLOPE
= 5.8
⋅
can be calculated according to equation 6.
SLOPE
10
10
s
-1
between the SLOPE pin and the output. The compensation ramp
L
, V
dies away in subsequent cycles thanks to the slope compensation
BE
= 2 V (typ.) and S
270mV threshold
Figure 12
S
E
=
K
S
⎛
⎜
⎜
⎝
, where the switching duty-cycle is higher than 0.5,
V
OUT
R
−
SLOPE
E
V
is the slope ramp in [A/s].
IN
270ns minimum
−
V
BE
⎞
⎟
⎟
⎠
LED7707
AM00588v1