SGM3122 Datasheet, PDF (9/17 Pages) SG Micro Corp – PWM Brightness Control

SGM3122
White LED Charge Pump Current
Source with PWM Brightness Control
DETAILED DESCRIPTION
Operation
The SGM3122 uses a fractional conversion charge pump
to generate a supply voltage for the integrated current
sinks. These current sinks are used to ensure a constant
current for each LED. Depending on the input voltage
and programmed LED current, the charge pump either
operates in the 1× mode or in the 1.5× mode. By
switching automatically between these two modes, the
circuit optimizes power conversion efficiency as well as
extends operating time by allowing the discharge of the
battery completely.
The charge pump can generate 120mA of output current,
so each of the 4-LED outputs can be powered with up to
30mA of current. The maximum LED current is set by a
resistor connected to the ISET pin. This resistor
programs a reference current, which is current mirrored
to set the LED current.
Applying a PWM signals to the EN1 pin and/or the EN2
pin controls the LED brightness. See a detailed
description in the section Analog Dimming Using ISET
Pin.
LED Current Adjustment (ISET)
A resistor programs a reference current, which is current
mirrored to set the LED current. The voltage at the ISET
pin depends on the status of EN1 and EN2. The current
in each LED is typically 255 times the current through the
resistor at ISET (see Table 2).
R_ISET =
V_ISET
I_LED
×K
V_ISET — Voltage from ISET pin (0.2V, 0.4V or 0.6V) to
GND, see Table 1.
I_LED — Current per LED from D_X pin to GND.
K — D_X to ISET current ratio (typically 255).
The LED current varies linearly from 0mA to I_LED (max) by
applying a PMW signal with 0% to 100% duty cycle.
The LED brightness can however also be controlled by
an analog control signal that is fed into the ISET pin.
Soft-Start
The SGM3122 has an internal soft-start circuit to limit the
inrush current during startup. This prevents possible
voltage drops of the input voltage if a high impedance
power source is connected to the input of the SGM3122.
When the device starts up with an output voltage that is
below the input voltage, the output capacitor is charged
directly from the input with a current source. The output
current increases linearly until the output reaches within
300mV of the input voltage. When the programmed
output current can be reached with the 1× mode, the
SGM3122 terminates the soft-start and begins normal
operation. When the desired output current cannot be
reached, the charge pump begins operation in 1.5× mode
and pumps the output voltage up to the needed level to
reach the programmed output current.
Enable (EN1, EN2)
The enable pins EN1 and EN2 are used to enable the
device or set it into shutdown. The SGM3122 is enabled
if one of the enable pins is pulled higher than the enable
trip point of 1.3V. The device starts up by going through
the soft-start routine as described in the section
Soft-Start. Pulling both pins to GND, after a delay,
programs the device to shutdown. In shutdown, the
charge pump, current sources, voltage reference,
oscillator, and all other functions are turned off and the
supply current is reduced to 0.1µA.
EN1 and EN2 can also be used for dimming. The logic
levels at EN1 and EN2 set the minimum voltage at the
current mirrors and the voltage at the ISET pin to GND.
This sets the current at the LEDs to be either the full
current or a fraction of the full current. See Table 1 for
further details. The maximum current through the LEDs is
set by a resistor connected between ISET and GND.
EN1 and EN2 can also be used for PWM dimming. The
PWM signal can either be applied to EN1 or EN2, or both
inputs can be tied together and the PWM signal can be
applied to both pins. Depending on the configuration, the
current during PWM dimming is switched between 0mA
and its maximum (EN1 and EN2 connected to the PWM
signal) or between 0mA and 1/3 of the full LED current if
EN2 = 0 and EN1 is toggled. When EN1 = 0 and EN2 is
toggled, the output current can be changed between 0mA
and 2/3 of the full range.
SG Micro Corp
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