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PDF ACT6358 Data sheet ( Hoja de datos )
| Número de pieza | ACT6358 | |
| Descripción | (ACT6357 / ACT6358) 40V Step-Up WLED Bias Supplies | |
| Fabricantes | Active-Semi | |
| Logotipo |  |
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ACT6357/ACT6358www.DataSheet4U.com
Rev0, 03-Feb-08
Advanced Product Information―All Information Subject to Change
High-Efficiency, 40V Step-Up WLED Bias Supplies
FEATURES
• High-Efficiency DC/DC WLED Bias Supply
• Internal 40V, 0.55Ω Power MOSFET
• Up to 10 WLEDs per String
• Two Peak Current Options:
− ACT6357: 0.5A
− ACT6358: 1A
• Supports Analog and PWM LED Dimming
• Integrated Over-Voltage Protection (OVP)
• Programmable Soft-Start Function
• Thermal Shutdown
• Cycle-by-Cycle Over Current Protection
• Tiny TDFN33-8 Package
APPLICATIONS
• TFT LCD Displays
• Smart Phones
• Portable Media Players
• GPS/Personal Navigation Devices
GENERAL DESCRIPTION
The ACT6357 and ACT6358 step-up DC/DC con-
verters drive white LEDs with an externally pro-
grammable constant current. These devices feature
integrated, 40V power MOSFETs that are capable
of driving up to ten white LEDs in series, providing
inherent current matching for uniform brightness.
WLED brightness adjustment is easily achieved via
a dual-function pin, which accepts either a PWM or
an analog dimming control signal.
The ACT6357 and ACT6358 feature a variety of
protection circuits, including integrated over voltage
protection (OVP), programmable soft-start, cycle-
by-cycle current limiting, and thermal shutdown pro-
tection circuitry.
The ACT6357 has 500mA current limit, while the
ACT6358 has 1A current limit. Both parts are avail-
able in a small 3mm x 3mm 8-pin TDFN33-8.
SIMPLIFIED APPLICATION CIRCUIT
L1
VIN
2.6V to 5.5V
CIN
Enable
Brightness Control
CSS
Up to
D1 10 WLEDs
IN SW
OUT
ROV2
ACT6357 OV
EN ACT6358
ROV1
BC
SS
G
FB
RFB
COUT
Innovative Products. Active Solutions.
-1-
www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
1 page  
FUNCTIONAL BLOCK DIAGRAM
ACT6357/ACT6358www.DataSheet4U.com
Rev0, 03-Feb-08
Control Scheme
The ACT6357 and ACT6358 use a minimum off-
time, current-mode control scheme to achieve ex-
cellent performance under high duty-cycle operating
conditions. This control scheme initiates a switching
cycle only when needed to maintain output voltage
regulation, resulting in very high efficiency opera-
tion.
During each switching cycle, the N-channel power
MOSFET turns on, increasing the inductor current.
The switching cycle terminates when either the in-
ductor current reaches the current limit (500mA for
the ACT6357, 1A for the ACT6358) or when the
cycle lasts longer than the maximum on-time of
4µs. Once the MOSFET turns off, it remains off for
at least the minimum off-time of 320ns, then an-
other switching begins when the error comparator
detects that the output is falling out of regulation
again.
Soft-Start
The ACT6357 and ACT6358 include a programma-
ble soft-start function, which can be used to opti-
mize an application between start-up time and
start-up inrush current. Soft start is achieved by
connecting a capacitor CSS between the SS pin and
G. The soft start duration can be calculated from
the following equation:
CSS
= tSS
× 5 µF
s
where tSS is the required soft start duration. In a
typical application, use 0.1µF to generate 20ms soft
start time.
Over Voltage Protection
Both the ACT6357 and ACT6358 include internal
over-voltage protection circuitry that monitors the
OV pin voltage. Over-voltage protection is critical
when one of the LEDs in the LED string fails as an
open circuit. When this happens the feedback volt-
age drops to zero, and the control switches at maxi-
mum on time causing the output voltage to keep
rising until it exceeds the maximum voltage rating of
the power MOSFET. The ACT6357 and ACT6358's
over-voltage protection detects this condition and
switching ceases if the voltage at the OV pin
reaches 1.21V.
To set the maximum output voltage, connect a re-
sistor divider from the output node to G, with center
tap at OV, and select the two resistors with the fol-
lowing equation:
ROV2
=
ROV1
× ⎢⎣⎡⎜⎝⎛1V.2O1VV
⎟⎞
⎠
−1⎥⎦⎤
where VOV is the over voltage detection threshold,
ROV1 is the resistor between OV and G, and ROV2 is
the resistor from the output to the OV pin. As a first
estimate, the OV threshold can often be set to 4V
times the number of LEDs in the string.
Innovative Products. Active Solutions.
-5-
www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
5 Page | ||
| Páginas | Total 10 Páginas | |
| PDF Descargar | [ Datasheet ACT6358.PDF ] | |
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