MAX8595X/MAX8596X
Detailed Description
The high efficiency and small size of the MAX8595X/
MAX8596X make them ideally suited to drive up to nine
series-connected LEDs. These devices operate as a
boost DC-DC converter that regulates output current
rather than voltage. The MAX8595X/MAX8596X provide
even illumination by sourcing the same output current
through each LED, eliminating the need for expensive
factory calibration. The fast 1MHz internal oscillator
allows for a small inductor and small input and output
capacitors while minimizing input and output ripple.
The single analog control input (CTRL) allows easy
adjustment of LED brightness and on/off control. This
allows simple logic-level on/off control, analog voltage
control, or PWM duty-cycle control of both brightness
and shutdown. In shutdown, supply current is reduced
to a low 0.3µA (typ). A soft-start gradually illuminates
the LEDs, eliminating the inrush current during startup.
The MAX8596X has the additional feature of derating
LED current as ambient temperature rises. Above
+42°C, the CS regulation voltage is reduced at a rate of
5.5mV/°C, thus reducing the LED current.
Soft-Start
The MAX8595X/MAX8596X attain soft-start by charging
C
COMP
gradually with a current source. When V
COMP
rises above 1.25V, the internal MOSFET begins switch-
ing at a reduced duty cycle. When V
COMP
rises above
2.25V, the duty cycle is at its maximum. See the
Typical Operating Characteristics for an example of
soft-start operation.
Shutdown
The MAX8595X/MAX8596X enter shutdown when V
CTRL
is less than 100mV for more than 8.2ms. In shutdown,
supply current is reduced to 0.3µA (typ) by powering
down the entire IC except for the CTRL voltage-detec-
tion circuitry. C
COMP
is discharged during shutdown,
allowing the device to reinitiate soft-start when it is
enabled. Although the internal n-channel MOSFET does
not switch in shutdown, there is still a DC current path
between the input and the LEDs through the inductor
and Schottky diode. The minimum forward voltage of the
LED array must exceed the maximum input voltage to
ensure that the LEDs remain off in shutdown. However,
with two or more LEDs, the forward voltage is large
enough to keep leakage current low, less than 1µA
(typ). Typical shutdown timing characteristics are shown
in the Typical Operating Characteristics.
Overvoltage Protection
Overvoltage lockout (OVLO) occurs when V
OUT
is
above 38V (typ). The protection circuitry stops the inter-
nal MOSFET from switching and causes V
COMP
to
decay towards 0V. The device comes out of OVLO and
into soft-start when V
OUT
falls below 36V (typ).
Ambient Temperature Derating Function
(MAX8596X)
The MAX8596X limits the maximum LED current
depending on the die temperature. V
CS
is limited to
343mV up to +42°C. Once the temperature reaches
+42°C, the maximum V
CS
declines by 5.5mV/°C until
the minimum 106.5mV threshold is reached at +85°C.
Due to the package’s exposed paddle, the die temper-
ature is always very close to the PC board temperature.
The temperature derating function allows the LED cur-
rent to be safely set higher at normal operating temper-
atures, thereby allowing either a brighter display or
fewer LEDs to be used for normal display brightness.
See the Typical Operating Characteristics for LED
Current vs. Ambient Temperature.
High-Efficiency, 36V Step-Up Converters
with T
A
Derating Option for 2 to 9 White LEDs
6 _______________________________________________________________________________________
