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NCP1030, NCP1031
OPERATING DESCRIPTION
cos --1 ? 1 ? DC OUT ? ? N P ? × ? L OUT C OUT ? Ibias
CCC =
Introduction
The NCP1030 and NCP1031 are a family of miniature
monolithic voltage--mode switching regulators designed for
isolated and non--isolated bias supply applications. The
Forward:
V N
? V in S
2.6
(eq. 1)
internal startup circuit and the MOSFET are rated at 200 V,
making them ideal for 48 V telecom and 42 V automotive
applications. In addition, the NCP103x family can operate
from an existing 12 V supply. This controller family is
optimized for operation up to 1 MHz.
The NCP103x family incorporates in a single IC all the
active power, control logic and protection circuitry required
to implement, with a minimum of external components,
several switching regulator applications, such as a
secondary side bias supply or a low power dc--dc converter.
The NCP1030 is available in the space saving Micro8 t
where, I bias is the bias current supplied by the V CC capacitor
including the IC bias current (I CC1 ) and any additional
current used to bias the feedback resistors (if used).
After initial startup, the V CC pin should be biased above
V CC(off) using an auxiliary winding. This will prevent the
startup regulator from turning ON and reduce power
dissipation. Also, the load should not be directly connected
to the V CC capacitor. Otherwise, the load may override the
startup circuit. Figure 33 shows the recommended
configuration for a non--isolated flyback converter.
package and is targeted for applications requiring up to 3 W.
The NCP1031 is targeted for applications up to 6 W and is
available in the SO--8 package.
The NCP103x includes an extensive set of features
including over temperature protection, cycle by cycle
current limit, individual line under and overvoltage
detection comparators with hysteresis, and regulator output
undervoltage lockout with hysteresis, providing full
protection during fault conditions. A description of each of
the functional blocks is given below, and the representative
block diagram is shown in Figure 2.
Startup Circuit and Undervoltage Lockout
The NCP103x contains an internal 200 V startup regulator
that eliminates the need for external startup components.
+
V in
--
NCP103x
GND V DRAIN
C T V CC
V FB UV
COMP OV
+
V out
--
The startup regulator consists of a constant current source
that supplies current from the input line (V in ) to the capacitor
on the V CC pin (C CC ). Once the V CC voltage reaches
approximately 10 V, the startup circuit is disabled and the
Power Switch Circuit is enabled if no faults are present.
During this self--bias mode, power to the NCP103x is
supplied by the V CC capacitor. The startup regulator turns
ON again once V CC reaches 7.5 V. This “7.5--10” mode of
operation is known as Dynamic Self Supply (DSS). The
NCP1030 and NCP1031 startup currents are 12 mA and 16
mA, respectively.
If V CC falls below 7.5 V, the device enters a re--start mode.
While in the re--start mode, the V CC capacitor is allowed to
discharge to 6.5 V while the Power Switch is enabled. Once
the 6.5 V threshold is reached, the Power Switch Circuit is
disabled and the startup regulator is enabled to charge the
V CC capacitor. The Power Switch is enabled again once the
V CC voltage reaches 10 V. Therefore, the external V CC
capacitor must be sized such that a voltage greater than 7.5
V is maintained on the V CC capacitor while the converter
output reaches regulation. Otherwise, the converter will
enter the re--start mode. Equation (1) provides a guideline
for the selection of the V CC capacitor for a forward
converter;
Figure 33. Non--Isolated Bias Supply Configuration
The maximum voltage rating of the startup circuit is
200 V. Power dissipation should be observed to avoid
exceeding the maximum power dissipation of the package.
Error Amplifier
The internal error amplifier (EA) regulates the output
voltage of the bias supply. It compares a scaled output
voltage signal to an internal 2.5 V reference (V REF )
connected to its non--inverting input. The scaled signal is fed
into the feedback pin ( V FB ) which is the inverting input of the
error amplifier.
The output of the error amplifier is available for frequency
compensation and connection to the PWM comparator
through the COMP pin. To insure normal operation, the EA
compensation should be selected such that the EA frequency
response crosses 0 dB below 80 kHz.
The error amplifier input bias current is less than 1 m A
over the operating range. The output source and sink
currents are typically 110 m A and 550 m A, respectively.
Under load transient conditions, COMP may need to
move from the bottom to the top of the C T Ramp. A large
current is required to complete the COMP swing if small
resistors or large capacitors are used to implement the
compensation network. In which case, the COMP swing will
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