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

rises above 1.25V, the internal MOSFET begins switch-

2.25V, the duty cycle is at its maximum. See the

Typical Operating Characteristics for an example of

soft-start operation.

Shutdown

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-

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

above 38V (typ). The protection circuitry stops the inter-

decay towards 0V. The device comes out of OVLO and

Ambient Temperature Derating Function

(MAX8596X)

The MAX8596X limits the maximum LED current

343mV up to +42

°C. Once the temperature reaches

+42

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

6

_______________________________________________________________________________________

0.1

µF

PWM

CONTROL

IN

2.2

µF

22

µH

LX

PGND

OVER-

VOLTAGE

PROTECT

CS

SHUTDOWN

8.2ms

GND

COMP

CTRL

170mV

100k

Ω

121k

Ω

279k

Ω

1.25V CLAMP OR

TEMP DERATE CLAMP

13

Ω

2.6V TO 5.5V

1MHz

0.1

µF

ANALOG

OR DIRECT

PWM

DIMMING

OUT

Figure 1. Functional Diagram and Typical Application Circuit