LT1683 查看數據表（PDF） - Linear Technology

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LT1683

Slew Rate Controlled Ultralow Noise Push-Pull DC/DC Controller

Linear Technology 

LT1683

OPERATION

The voltage slew rate is thus inversely proportional to both

the value of capacitor and RVSL.

The current slew feedback loop consists of the voltage

across the external sense resistor, which is internally ampli-

fied and differentiated. The derivative is limited to a value set

by RCSL. The current slew rate is thus inversely proportional

to both the value of sense resistor and RCSL.

The two control loops are combined internally so that a

smooth transition from current slew control to voltage

slew control is obtained. When turning on, the driver cur-

rent will slew before voltage. When turning off, voltage

will slew before current. In general it is desirable to have

RVSL and RCSL of similar value.

Internal Regulator

Most of the control circuitry operates from an internal

2.4V low dropout regulator that is powered from VIN. The

internal low dropout design allows VIN to vary from 2.7V

to 20V with stable operation of the controller. When SHDN

< 1.3V the internal regulator is completely disabled.

5V Regulator

A 5V regulator is provided for powering external circuitry.

This regulator draws current from VIN and requires VIN

to be greater than 6.5V to be in regulation. It can sink or

source 10mA. The output is current limited to prevent

against destruction from accidental short circuits.

Safety and Protection Features

There are several safety and protection features on the

chip. The first is overcurrent limit. Normally the gate

drivers will go low when the output of the internal sense

amplifier exceeds the voltage on the VC pin. The VC pin is

clamped such that maximum output current is attained

when the CS pin voltage is 0.1V. At that level the outputs

will be immediately turned off (no slew). The effect of

this control is that the output voltage will foldback with

overcurrent.

In addition, if the CS voltage exceeds 0.22V, the VC and

SS pins will be discharged to ground also, resetting the

soft-start function. Thus if a short is present this will allow

for faster MOSFET turnoff and less MOSFET stress.

If the voltage on the FB pin exceeds regulation by ap-

proximately 0.22V, the outputs will immediately go low.

The implication is that there is an overvoltage fault.

The voltage on GCL determines two features. The first

is the maximum gate drive voltage. This will protect the

MOSFET gate from overvoltage.

With GCL tied to a Zener or an external voltage source

then the maximum gate driver voltage is approximately

VGCL – 0.2V. If GCL is tied to VIN, then the maximum

gate voltage is determined by VIN and is approximately

VIN – 1.6V. There is an internal 19V Zener on the GCL

pin that prevents the gate driver pin from exceeding ap-

proximately 19V.

In addition, the GCL voltage determines undervoltage

lockout of the gate drives. This feature disables the gate

drivers if VIN is too low to provide adequate voltage to

turn on the MOSFETs. This is helpful during start-up to

ensure the MOSFETs have sufficient gate drive to saturate.

If GCL is tied to a voltage source or Zener less than 6.8V,

the gate drivers will not turn on until VIN exceeds GCL

voltage by 0.8V. For VGCL above 6.5V, the gate drives are

ensured to be off for VIN < 7.3V and they will be turned

on by VGCL + 0.8V.

If GCL is tied to VIN, the gate drivers are always enabled

(undervoltage lockout is disabled).

When driving a push-pull transformer, it is important to

make sure that both drivers are not on at the same time.

Even though runaway cannot occur under such cross

conduction with this chip because current slew is regu-

lated, increased current would be possible. This chip has

opposite gate lockout whereby when one MOSFET is on

the other MOSFET cannot be turned on until the gate of

the first drops below 1V. This ensures that cross conduc-

tion will not occur.

The gate drives have current limits for the drive currents.

If the sink or source current is greater than 300mA then

the current will be limited.

The V5 regulator also has internal current limiting that will

only guarantee ±10mA output current.

1683fd

11

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