MAX8595XETA-T 查看數據表（PDF） - Maxim Integrated

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MAX8595XETA-T

High-Efficiency, 36V Step-Up Converters with TA Derating Option for 2 to 9 White LEDs

Maxim Integrated 

High-Efficiency, 36V Step-Up Converters

with TA Derating Option for 2 to 9 White LEDs

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

CCOMP gradually with a current source. When VCOMP

rises above 1.25V, the internal MOSFET begins switch-

ing at a reduced duty cycle. When VCOMP 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 VCTRL

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. CCOMP 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 VOUT is

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

2.6V TO 6.0V

2.2µF

PWM

CONTROL

fOSC

1MHz

0.1µF

COMP

IN

LX

PGND

OUT

OVER-

VOLTAGE

PROTECT

1.25V CLAMP OR

TEMP DERATE CLAMP

gm

CTRL

ANALOG

121kΩ 279kΩ

OR DIRECT

100kΩ

CS

PWM

DIMMING

170mV

8.2ms SHUTDOWN

22µH

0.1µF

RSENSE

13Ω

GND

Figure 1. Functional Diagram and Typical Application Circuit

nal MOSFET from switching and causes VCOMP to

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

into soft-start when VOUT falls below 36V (typ).

Ambient Temperature Derating Function

(MAX8596X)

The MAX8596X limits the maximum LED current

depending on the die temperature. VCS is limited to

343mV up to +42°C. Once the temperature reaches

+42°C, the maximum VCS 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.

6 _______________________________________________________________________________________

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