Datasheet

Output Current Limiting
Current limiting inception is defi ned as the point at which full power falls
below the rated tolerance. See the Performance/Functional Specifi ca-
tions. Note particularly that the output current may briefl y rise above its
rated value in normal operation as long as the average output power is
not exceeded. This enhances reliability and continued operation of your
application. If the output current is too high, the converter will enter the
short circuit condition.
Output Short Circuit Condition
When a converter is in current-limit mode, the output voltage will drop
as the output current demand increases. If the output voltage drops too
low (approximately 98% of nominal output voltage for most models), the
magnetically coupled voltage used to develop primary side voltages will
also drop, thereby shutting down the PWM controller. Following a time-out
period, the PWM will restart, causing the output voltage to begin ramping
up to its appropriate value. If the short-circuit condition persists, another
shutdown cycle will initiate. This rapid on/off cycling is called “hiccup
mode”. The hiccup cycling reduces the average output current, thereby
preventing excessive internal temperatures and/or component damage. A
short circuit can be tolerated indefi nitely.
BOM  Rp  20K
BOM  Q1  Q SMT MOS P 30V
GND GND
+Vin +Vin
Rp
Q1
R1
E
PWM
OKL P Module
On/Off
Figure 6. On/Off Circuit Control for Using Positive On/Off Logic
BOM  Rp  20K
BOM  Q1  Q SMT MOS P 30V
GND GND
+Vin +Vin
Rp
Q1
Q3
E
GND
OKL N Module
On/Off
PWM
Figure 7. On/Off Circuit Control for Using Negative On/Off Logic
The “hiccup” system differs from older latching short circuit systems
because you do not have to power down the converter to make it restart.
The system will automatically restore operation as soon as the short circuit
condition is removed.
Remote On/Off Control
The OKL Series power modules can be specifi ed with either a positive or nega-
tive logic type. See Figures 6 and 7 for On/Off circuit control. In the positive logic
on/off option the unit turns on during a logic high on the On/Off pin and turns off
during a logic low. In a negative logic on/off option, the unit turns off during logic
high and on during logic low. The On/Off signal should always be reference to
ground. For positive or negative option, leaving then On/Off pin disconnected
will turn the unit on when input voltage is present.
Positive—Units are enabled when the on/off pin is left open or is pulled
high to +Vin. The On/Off circuit control is shown in fi gure 6. When the
external transistor Q1 is in the off state, the internal PWM enable pin is pull
high causing the unit to turn on. When Q1 is turn on, the On/Off pin is pulled
low and the units is off. Rp should be around 20K ohms.
Negative—Units are enabled when the ON/Off is open or brought to
within a low voltage (see specifi cations) with respect to –Vin. The unit is off
when the ON/Off is pulled high with respect to –Vin (see specifi cations). The
On/Off circuitry is shown in fi gure 7. The On/Off pin should be pulled high
with an external pull-up resistor (20K ohms). When Q1 is in the off state,
the On/Off pin is pulled high, transistor Q3 is turn on and the unit is off. To
turn on the unit, Q1 is turn on, pulling the On/Off pin low and turning Q3 off
resulting on the unit being on.
Dynamic control of the On/Off function should be able to sink the speci-
fi ed signal current when brought low and withstand appropriate voltage
when brought high. Be aware too that there is a fi nite time in milliseconds
(see specifi cations) between the time of On/Off Control activation and
stable, regulated output. This time will vary slightly with output load type
and current and input conditions.
Output Capacitive Load
These converters do not require external capacitance added to achieve
rated specifi cations. Users should only consider adding capacitance to
reduce switching noise and/or to handle spike current load steps. Install
only enough capacitance to achieve noise objectives. Excess external
capacitance may cause regulation problems, degraded transient response
and possible oscillation or instability.
Synchronization
These converters can be synchronized using an external signal.
Details of the SYNC signal are provided in the Performance and Func-
tional Specifi cations table.
If the synchronization function is not being used, leave the SYNC pin
fl oating.
When synchronization function is used, output ripple may increase on
some operating conditions. Please check the proper operation of this device
with the peripheral circuits on your system.
OKL2-T/12-W12 Series
Programmable Output 12-Amp iLGA SMT PoL
DC-DC Converter Series
MDC_OKL2-T/12-W12.B02 Page 17 of 19
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