®

LSN2 Series

Non-isolated, DOSA-SIP, 6/10/16A Selectable-Output DC/DC Converters

www.cd4power.com

Figure 5. Inverting On/Off Control

Figure 4. On/Off Control Using An External Open Collector Driver

The internal 10.5

Ω resistor between +Sense and +Output (see Figure 1)

serves to protect the sense function by limiting the output current flowing

through the sense line if the main output is disconnected. It also prevents

output voltage runaway if the sense connection is disconnected.

Note: If the sense function is not used for remote regulation, +Sense

must be tied to +Output at the DC/DC converter pins.

Remote On/Off Control

The input-side remote On/Off Control is an external input signal available in

either positive (no suffix) or negative (“N” suffix) polarity. Normally this input

is controlled by the user’s external transistor or relay. With simple external

circuits, it may also be selected by logic outputs. Please note however that

the actual control threshold levels vary somewhat with the PWM supply and

therefore are best suited to “open collector” or “open drain” type logic. The

On/Off control takes effect only when appropriate input power has been

applied and stabilized (approximately 7msec).

For positive polarity, the default operation leaves this pin open (unconnected)

or HIGH. The output will then always be on (enabled) whenever appropriate

input power is applied. Negative polarity models require the On/Off to be

grounded to the –Input terminal or brought LOW to turn the converter on.

To turn the converter off, for positive polarity models, ground the On/Off

control or bring it LOW. For negative polarity, raise the On/Off at least to +2.5V

to turn it off.

Dynamic control of the On/Off must be capable of sinking or sourcing the

control current (approximately 1mA max.) and not overdrive the input greater

activating the On/Off control. Be aware that a delay of several milliseconds

occurs (see specifications) between activation of the control and the resulting

change in the output.

Power-up sequencing

If a controlled start-up of one or more LSN2 Series DC/DC converters is

required, or if several output voltages need to be powered-up in a given

sequence, the On/Off control pin can be driven with an external open collector

device as per Figure 4.

Leaving the input of the on/off circuit closed during power-up will have the

output of the DC/DC converter disabled. When the input to the external open

collector is pulled high, the DC/DC converter's output will be enabled.

Output Overvoltage Protection

LSN2 SIP Series DC/DC converters do not incorporate output overvoltage pro-

tection. In the extremely rare situation in which the device’s feedback loop is

is absolutely imperative that you protect your load against any and all possible

overvoltage situations, voltage limiting circuitry must be provided external to

the power converter.

Output Overcurrent Detection

Overloading the power converter's output for an extended time will invariably

cause internal component temperatures to exceed their maximum ratings and

eventually lead to component failure. High-current-carrying components such

as inductors, FET's and diodes are at the highest risk. LSN2 SIP Series DC/DC

converters incorporate an output overcurrent detection and shutdown function

that serves to protect both the power converter and its load.

If the output current exceeds it maximum rating by typically 50% or if the

output voltage drops to less than 98% of it original value, the LSN2's internal

overcurrent-detection circuitry immediately turns off the converter, which then

goes into a "hiccup" mode. While hiccupping, the converter will continuously

attempt to restart itself, go into overcurrent, and then shut down. Once the

output short is removed, the converter will automatically restart itself.

Output Reverse Conduction

Many DC/DC's using synchronous rectification suffer from Output Reverse

Conduction. If those devices have a voltage applied across their output before

a voltage is applied to their input (this typically occurs when another power

supply starts before them in a power-sequenced application), they will either

fail to start or self destruct. In both cases, the cause is the "freewheeling" or

"catch" FET biasing itself on and effectively becoming a short circuit.

LSN2 SIP DC/DC converters do not suffer from Output Reverse Conduction.

They employ proprietary gate drive circuitry that makes them immune to

moderate applied output overvoltages.

Thermal Considerations and Thermal Protection

The typical output-current thermal-derating curves shown below enable

designers to determine how much current they can reliably derive from each

model of the LSN2 SIPs under known ambient-temperature and air-flow con-

ditions. Similarly, the curves indicate how much air flow is required to reliably

deliver a specific output current at known temperatures.