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KB4312 Schematic ( PDF Datasheet ) - Kingbor Technology

Teilenummer KB4312
Beschreibung Constant Current Switching Regulator
Hersteller Kingbor Technology
Logo Kingbor Technology Logo 




Gesamt 9 Seiten
KB4312 Datasheet, Funktion
Kingbor Technology Co.,Ltd
TEL:(86)0755-26508846 FAX:(86)0755-26509052
KB4312
Very Low
Cost
Tiny Package, High Performance, Constant Current
Switching Regulator for White LED
FEATURES
•Operating Voltage:2.5V~6.5V
•High Operating Frequency: 1.2MHz
•High Efficiency: Up to 92%
•High Output Voltage: Up to 28V(max)
•Shutdown Current <1PA
•18V Output Over-voltage protection (ADJ)
•Digital Dimming Control.
•Built-in Cycle-by Cycle Current-limiting.
•Built-in Soft-Start Function.
•0.24V Low Reference Voltage(+/-5%)
•Tiny SOT23 Package
APPLICATIONS
• LED Module
• LED Backlight for LCD Display (1~6.5 )
• Constant Current Source!
DESCRIPTION
The KB4312 step-up converter drives white LEDs
with a constant current to provide backlight in cell
phones, PDAs, and other hand-held devices. It
features allowing series connection of the white
LEDs so that the LED currents are identical for
uniform brightness. An enable input can be pulsed
repeatedly to adjust LED’s brightness. The fast 1.2
MHz current-mode PWM control allows for smaller
capacitor and inductor. Fault condition protection
uses cycle-by-cycle current limiting to sense
maximum inductor current, thermal protection and
over-voltage protection. Also included soft-start
eliminates inrush current during start-up. The 0.24V
low reference voltage minimized the power loss
across the current-setting resistor.
The KB4312 is available in SOT23 packages /
SIMPLIFIED BLOC DIAGRAM
VIN
6
EN 4
ON/OFF
Control
Logic / PWM
Dimming
Control
CTL
VREF
0.240V
VBG
1.27V
Soft-Start
+
FB
OVP
3
5 17V
-
KB4312B FB=240mV
KB4312A FB=1.27V
VBG
+
Overvoltage
Protection
-
Control Logic
Current Limit
0.25A
+
-
N
CTL
2
GND
1 LX
REV: 3.2
1






KB4312 Datasheet, Funktion
Kingbor Technology Co.,Ltd
TEL:(86)0755-26508846 FAX:(86)0755-26509052
KB4312
Function Description
The KB4312 is a high efficiency and constant current DC-DC converter IC which is designed primarily
for use in LED module applications. The output voltage of the step-up converter can be set from Vin to
28V with external sense resistor. The device suits to drive series-connected LEDs and provides even
illumination by sourcing the same output current through each LED. KB4312 also consists of a logic
shutdown , cycle-by-cycle current-limited , soft-start functions.
Logic control input or PWM duty cycle control allows easy adjustment of LEDs brightness and on/off
control. The average LED current is proportionable the duty-cycle of the PWM signal. Typical PWM
frequency should be between 100Hz and 1kHz.
The boost converter operates in current-mode PWM and a constant frequency of 1.2 MHz. Depending
on duty cycle of each switching cycle can regulate output voltage. On the rising edge of the internal
clock , the control and driver logic block sets internal flip-flop when the output voltage is too low,
which turns on the N-MOS. The external inductor current ramps up linearly, storing energy in a
magnetic filed. Once peak current of inductor over trans-conductance output level , the N-MOS turns
off, the flip-flop resets, and external schottky diode turns on. This forces the current through the
inductor to ramp back down, transferring the energy stored in the magnetic field to the output capacitor
and LEDs. To reduce external component amount , the device will be built-in internal loop
compensation.
ʳ
Enable Control
Digital logic of EN provides an electrical ON/OFF control of the power supply. Connecting this pin to
ground or to any voltage less than 0.4V and sustain the level over 4ms will completely turn off the
regulator. In this state, current drain from the input supply is less than 1uA, the internal reference, error
amplifier, comparators, and biasing circuitry turn off . If holding time of low level is less than 3ms on
this pin, then the device only shutdown driver logic block.
ʳ
Dimming Control
Digital logic of EN also provides LEDs brightness control by applying a PWM signal on EN pin. With
this way, the LEDs operate with either zero or full current . The average LED current is proportional to
the duty-cycle of the PWM signal. Typical PWM frequency should be between 100Hz and 1kHz.
Output current is given by:
I LED
105mV u TON PWM ʳ
RSENSE u TS PWM
Where:
TON-PWM : On time of PWM signal
TS-PWM : A cycle time of PWM signal
If dimming control is not required , EN works as a simple on/off control. Drive EN high to enable the
device , or drive EN low for shutdown.
ʳ
6

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