LT3640IUFD View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3640IUFD

Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer

Linear Technology 

LT3640

Applications Information

Reverse Protection

In battery charging applications or in battery back-up

systems, the output will be held high when the input to the

LT3640 is absent. If the VIN pin is floated and the LT3640 is

enabled, the LT3640’s internal circuitry will pull its quiescent

current through the SW1 pin or the SW2 pin. This is fine if

the system can tolerate a few mA in this state. If the LT3640

is disabled, the SW1 pin and the SW2 pin current will drop

to essentially zero. However, if the VIN pin is grounded while

the high voltage channel output is held high, an external

diode is required at the VIN pin to prevent current being

pulled out of the VIN pin. If the VIN2 pin is grounded while

the low voltage channel output is held high, an external

diode is required at the VIN2 pin to prevent current being

pulled out of the VIN2 pin (Figure 7).

IN

VIN SW BST SW1

DA

FB1

EN/UVLO

LT3640

SW2

IN2

VIN2

FB2

OUT1

+

–

OUT2

+

–

GND

3640 F07

Figure 7. Diodes Prevent Shorted Inputs from

Discharging a Battery Tied to the Outputs

PFM Operation

To improve efficiency at light loads, the LT3640 auto-

matically switches to pulse frequency modulation (PFM)

operation which minimizes the switching loss and keeps

the output voltage ripples small.

Because the two channels of the LT3640 may have differ-

ent loads, the two channels can have different switching

frequency (Figure 8).

Power-On Reset Timer

Each channel of the LT3640 has a power-on comparator. Both

comparators are enabled when the LT3640 is powered up

and starts monitoring their corresponding feedback voltages.

18

SW1

10V/DIV

IL1

0.5A/DIV

SW2

5V/DIV

IL2

0.5A/DIV

500ns/DIV

3640 F08a

VIN = 12V

VIN2 = VOUT1

VOUT1 = 3.3V/25mA VOUT2 = 1.8V/30mA

(8a)

SW1

10V/DIV

IL1

0.5A/DIV

SW2

5V/DIV

IL2

0.5A/DIV

2µs/DIV

3640 F08b

VIN = 12V

VIN2 = VOUT1

VOUT1 = 3.3V/25mA VOUT2 = 1.8V/20mA

(8b)

SW1

10V/DIV

IL1

0.5A/DIV

SW2

5V/DIV

IL2

0.5A/DIV

2µs/DIV

3640 F08c

VIN = 12V

VIN2 = VOUT1

VOUT1 = 3.3V/0mA VOUT2 = 1.8V/30mA

(8c)

Figure 8. PFM Operation

The threshold of power-on comparator is 1.15V for the high

voltage channel, and 550mV for the low voltage channel.

Both RST1 and RST2 are open-drain outputs with weak

internal pull-ups (100k to ~2V). The DC characteristics of

the RST1 and RST2 pull-down strength are shown in the

Typical Performance Characteristics section. The weak

3640f

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