NCP1445 ONSEMI [ON Semiconductor], NCP1445 Datasheet - Page 11

• Manufacturer Part Number: NCP1445
• Description: 4.0 A 280 kHz/560 kHz Boost Regulators
• Manufacturer: ONSEMI [ON Semiconductor]

Switch Driver and Power Switch
section to drive the output power switch. The switch is
grounded through emitter resistors (15 mW 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 (V
voltage of the switch is typically less than 1.0 V to minimize
power dissipation.
Short Circuit Condition
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.
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.
V
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.
voltage, the V
current conducts directly from the input power source to the
CC
The switch driver receives a control signal from the logic
When a short circuit condition happens in a boost circuit,
In other topologies, the frequency shift built into the IC
The NCP144X can be activated by either connecting the
When the V
Figure 29. Startup Waveforms of Circuit Shown in
pin to a voltage source or by enabling the SS pin.
the Application Diagram. Load = 400 mA.
CC
SW
voltage is below the minimum supply
pin is in high impedance. Therefore,
NCP1442, NCP1443, NCP1444, NCP1445
SW
pin). The saturation
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V
V
I
V
L
OUT
CC
C
11
output through the inductor and diode. Once V
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.
internal power switch starts to switch and a voltage pulse can
be seen at the V
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.
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.
Frequency Compensation
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.
DC accuracy over line and load variations. The DC gain of
a transconductance error amplifier can be calculated as
follows:
where:
When the V
When the FB pin voltage rises above 0.4 V, the frequency
The goal of frequency compensation is to achieve
The high DC gain in Figure 31 is desirable for achieving
G
R
O
M
Figure 30. A Typical Compensation Network
= error amplifier output resistance
= error amplifier transconductance;
NCP1442/3/4/5
C
SW
COMPONENT SELECTION
pin voltage rises above the threshold, the
Gain DC + G M
pin. Detecting a low output voltage at the
GND
V
C
R O
R1
C1
1.0 MW.
CC
reaches
C2