NCP1253

http://onsemi.com

10

APPLICATION INFORMATION

Introduction

The NCP1253 implements a standard current mode

architecture where the switch−off event is dictated by the

peak current setpoint. This component represents the ideal

candidate where low part−count and cost effectiveness are

the key parameters, particularly in low−cost ac−dc adapters,

open−frame power supplies etc. Capitalizing on the

NCP1200 series success, the NCP1253 brings all the

necessary components normally needed in today modern

power supply designs, bringing several enhancements such

compensation: implementing peak current mode control

at a fixed 65 kHz or 100 kHz frequency, the NCP1253

offers an internal ramp compensation signal that can

easily by summed up to the sensed current. Sub harmonic

oscillations can thus be compensated via the inclusion of

a simple resistor in series with the current−sense

information.

power always represents a difficult exercise when the

controller draws a significant amount of current during

start−up. Thanks to its proprietary architecture, the

NCP1253 is guaranteed to draw less than 15

mA

maximum, easing the design of low standby power

adapters.

signal varies the pace at which the oscillator frequency is

modulated. This helps spreading out energy in conducted

noise analysis. To improve the EMI signature at low

power levels, the jittering will not be disabled in

frequency foldback mode (light load conditions).

pulses is not compatible with no−load/light−load standby

power requirements. To excel in this domain, the

controller observes the feedback pin and when it reaches

a level of 1.5 V, the oscillator then starts to reduce its

switching frequency as the feedback level continues to

decrease. When the feedback pin reaches 1.05 V, the peak

current setpoint is internally frozen and the frequency

continues to decrease. It can go down to 26 kHz (typical)

reached for a feedback level of 350 mV roughly. At this

point, if the power continues to drop, the controller enters

classical skip−cycle mode.

switch from being stressed upon start−up. In this

controller, the soft−start is internally fixed to 4 ms.

Soft−start is activated when a new startup sequence

occurs or during an auto−recovery hiccup.

When the voltage on this pin exceeds 25.5 V typical, the

pulses are immediately stopped and the part latches off.

recovers from a brown−out event, the circuit is reset and

the part enters a new start−up sequence.

over−load protections are difficult to implement when a

strong leakage inductance between auxiliary and power

windings affects the transformer (the aux winding level

does not properly collapse in presence of an output short).

Here, every time the internal 0.8 V maximum peak

current limit is activated, an error flag is asserted and a

time period starts, thanks to an internal timer. When the

fault is validated, all pulses are stopped and the controller

enters an auto−recovery burst mode, with a soft−start

sequence at the beginning of each cycle. As soon as the

fault disappears, the SMPS resumes operation. Please

note that some version offers an auto−recovery mode as

we just described, some do not and latch off in case of a

short circuit.

Start−up Sequence

The NCP1253 start−up voltage is made purposely high to

This helps to operate with a small start−up current which,

start−up time. To further reduce the standby power, the

start−up current of the controller is extremely low, below

15

mA. The start−up resistor can therefore be connected to

the bulk capacitor or directly to the mains input voltage if

you wish to save a few more mW.