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8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability
Dimming Control
The MAX17105 has two dimming modes that are select-
DPWM frequency is specified by an external resistor
connected from the DFSET pin to SGND:
ed by the DFSET pin as shown in Table 3. The LED on/
off DPWM frequency is generated by the internal oscil-
lator for SMBus mode, while LED is directly on/off by the
f DPWM =
200Hz O 250k I
R DFSET
I LED_MAX =
PWMI pin in direct-PWM mode. The DPWM frequency
is adjustable through an external setting resistor and
has 3% accuracy over the 200Hz to 2kHz range. In
SMBus mode, the duty cycle of this DPWM signal can
be controlled externally through two interfaces: PWM
and SMBus. The ISET pin sets the amplitude of the cur-
rent sources for each LED string (Figure 2). The internal
DPWM signal directly controls the duty cycle of these
current sources. The resulting current is chopped and
synchronized to the DPWM signal. When filtered by the
slow response time of the human eye, the overall bright-
ness is modulated in a consistent flicker-free manner.
Full-Scale LED and Low-Level LED Current
The full-scale LED current I ISET is determined by the
resistors connected from ISET to SGND:
20mA O 50k I
R ISET
The acceptable resistance range is 33.3k I < R ISET <
open, which corresponds to full-scale LED current of
30mA > I LED_MAX > 0mA. Connect ISET to SGND sets
the test mode for 0.3mA (typ) full-scale LED current.
DPWM Frequency Setting
The MAX17105 uses an internal DPWM signal to perform
dimming control while operating at SMBus mode. The
PWMO
The adjustable range for the DFSET resistor, R DFSET , is
from 10k I to 500k I , corresponding to the DPWM fre-
quency of 5kHz > f DPWM > 100Hz.
Dimming Control Interfaces for SMBus Mode
The MAX17105’s dimming control circuit consists of
two interfaces: PWM and SMBus. The block diagram of
these two input interfaces is shown in Figure 4. . The dim-
ming can be performed in three modes: PWM, SMBus,
or DPST. In PWM mode, the brightness is adjusted by
the PWM signal applied to the PWMI pin. In SMBus
mode, the brightness is adjusted by an I 2 C command
from an uplink processor through a 2-wire SMBus. In
DPST mode, the brightness is adjusted by the product
of the PWM duty cycle and SMBus command value. This
DPWM control provides a dimming range with 8-bit reso-
lution down to 0% and supports Intel DPST to maximize
battery life.
The SMBus interface can be used to adjust the dim-
ming, as well as shut down the MAX17105. Before
the MAX17105 receives a turn-on command from the
SMBus, it automatically remains off. In this low-power
state, most of the control circuits are turned off, and only
part of the LDO is active to provide a loosely regulated
output of approximately 4.5V on the V CC pin to power the
SMBus interface.
PWMI
BUFFER
A
D
P 1
1
MUX
DIGITAL
MULTIPLIER
MUX
DPWM
SETTING
SMBus AND PWM INPUT BLOCK
PWM_MD
PWM_SEL
BACKLIGHT
ON/OFF
SDA
SCL
IDENTIFICATION
REGISTER
0x03
FAULT/STATUS
REGISTER
0x02
DEVICE
CONTROL
REGISTER
0x01
SMBus
INTERFACE
BRIGHTNESS
CONTROL
REGISTER
0x00
Figure 4. MAX17105 PWM and SMBus Interface Circuit
18
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