NCP1561

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4

PIN DESCRIPTION

Pin

Name

Application Information

1

This pin is connected to the bulk DC input voltage supply. A constant current source supplies current from

range is 21.5 V to 150 V.

2

UV

Input supply voltage is scaled down and sampled by means of a resistor divider. The supply voltage must be

scaled such that the voltage on the UV pin is 1.54 V at the minimum input voltage.

3

RAMP_OUT

Internal Feedforward (FF) Ramp Output. This signal can be externally routed to the RAMP_IN pin for

voltage−mode control operation.

4

FF

5

CS

Overcurrent sense input. If the CS voltage exceeds 0.95 V or 1.15 V, the converter enters the Cycle by Cycle

or Cycle Skip current limit mode, respectively.

6

CSKIP

The capacitor connected to this pin sets the Cycle Skip period. Once a cycle skip fault is detected, the

capacitor connected to this pin is discharged. The capacitor is then charged with a constant current of 12

mA.

The cycle skip period expires, once the voltage on this capacitor reaches 2.0 V. A soft−start sequence follows

at the conclusion of the fault period.

7

A single external resistor between this pin and GND sets the fixed oscillator frequency.

8

An external resistor between this pin and GND sets the voltage on the Max DC Comparator inverting input.

The duty cycle is limited by comparing the voltage on the Max DC Comparator inverting input to the

Feedforward Ramp.

9

SS

An internal 6.0

mA current source charges the external capacitor connected to this pin. The duty cycle is

limited during startup by comparing the voltage on this pin to the Oscillator Ramp. The soft−start comparator

limits the duty cycle while the SS voltage is below 2.0 V.

10

The error signal from an external error amplifier is fed into this input and compared to the Feedforward Ramp.

A series diode and resistor offset the voltage on this pin before it is applied to the PWM Comparator inverting

input.

11

Precision 5.0 V reference output. Maximum output current is 6.0 mA.

12

RAMP_IN

This pin configures the NCP1561 for voltage or current−mode control. The internal Feedforward Ramp

(voltage−mode) or a signal proportional to the inductor current (current−mode) is fed into this input and

13

OUT2

Output 2.

14

GND

Control circuit ground.

15

OUT1

Output 1.

16

Positive input supply voltage. This pin is connected to an external capacitor for energy storage. An internal

supplied to the IC via this pin, by means of an auxiliary winding. The startup circuit is disabled if the voltage