NCP1444 datasheet(11/20 Pages) ONSEMI

Part #	NCP1444

Description	4.0 A 280 kHz/560 kHz Boost Regulators

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Manufacturer	ONSEMI [ON Semiconductor]

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NCP1442, NCP1443, NCP1444, NCP1445

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11

Switch Driver and Power Switch

The switch driver receives a control signal from the logic

section to drive the output power switch. The switch is

grounded through emitter resistors (15 m

W total) to the

GND pin. The peak switching current is clamped by an

internal circuit. The clamp current is guaranteed to be

greater than 4.0 A and varies with duty cycle due to slope

compensation. The power switch can withstand a maximum

voltage of 40 V on the collector (VSW pin). The saturation

voltage of the switch is typically less than 1.0 V to minimize

power dissipation.

Short Circuit Condition

When a short circuit condition happens in a boost circuit,

the inductor current will increase during the whole

switching cycle, causing excessive current to be drawn from

the input power supply. Since control ICs don’t have the

means to limit load current, an external current limit circuit

(such as a fuse or relay) has to be implemented to protect the

load, power supply and ICs.

In other topologies, the frequency shift built into the IC

prevents damage to the chip and external components. This

feature reduces the minimum duty cycle and allows the

transformer secondary to absorb excess energy before the

switch turns back on.

Figure 29. Startup Waveforms of Circuit Shown in

the Application Diagram. Load = 400 mA.

IL

VOUT

VC

VCC

The NCP144X can be activated by either connecting the

VCC pin to a voltage source or by enabling the SS pin.

Startup waveforms shown in Figure 29 are measured in the

boost converter demonstrated in the Block Diagram

(Figure 2). Recorded after the input voltage is turned on, this

waveform shows the various phases during the power up

transition.

When the VCC voltage is below the minimum supply

voltage, the VSW pin is in high impedance. Therefore,

current conducts directly from the input power source to the

output through the inductor and diode. Once VCC reaches

approximately 1.5 V, the internal power switch briefly turns

on. This is a part of the NCP144X’s normal operation. The

turn−on of the power switch accounts for the initial current

swing.

When the VC pin voltage rises above the threshold, the

internal power switch starts to switch and a voltage pulse can

be seen at the VSW pin. Detecting a low output voltage at the

FB pin, the built−in frequency shift feature reduces the

switching frequency to a fraction of its nominal value,

reducing the minimum duty cycle, which is otherwise

limited by the minimum on−time of the switch. The peak

current during this phase is clamped by the internal current

limit.

When the FB pin voltage rises above 0.4 V, the frequency

increases to its nominal value, and the peak current begins

to decrease as the output approaches the regulation voltage.

The overshoot of the output voltage is prevented by the

active pull−on, by which the sink current of the error

amplifier is increased once an overvoltage condition is

detected. The overvoltage condition is defined as when the

FB pin voltage is 50 mV greater than the reference voltage.

COMPONENT SELECTION

Frequency Compensation

The goal of frequency compensation is to achieve

desirable transient response and DC regulation while

ensuring the stability of the system. A typical compensation

network, as shown in Figure 30, provides a frequency

response of two poles and one zero. This frequency response

is further illustrated in the Bode plot shown in Figure 31.

NCP1442/3/4/5

Figure 30. A Typical Compensation Network

VC

GND

C1

R1

C2

The high DC gain in Figure 31 is desirable for achieving

DC accuracy over line and load variations. The DC gain of

a transconductance error amplifier can be calculated as

follows:

GainDC + GM

RO

where:

GM = error amplifier transconductance;

RO = error amplifier output resistance ≈ 1.0 M

W.

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