ncp1595 ON Semiconductor, ncp1595 Datasheet - Page 10

• Manufacturer Part Number: ncp1595
• Description: Current Mode Pwm Converter For Low Voltage Outputs
• Manufacturer: ON Semiconductor

Introduction
switching frequency at 1.0 MHz. The P−Channel main
switch is set by the positive edge of the clock cycle going
into the PWM latch. The main switch is reset by the
PWM latch in the following three cases:
switch will turn on and the inductor current will ramp down.
If the inductor current ramps down to zero before the
initiation of next clock cycle, the regulator runs at
discontinuous conduction mode (DCM). Otherwise the
regulator is at continuous conduction mode (CCM). The
N−Channel switch will turn off when the clock cycle starts.
The duty cycle is given by the ratio of output voltage to input
voltage. The duty cycle is allowed to go to 100% to increase
transient load response when going from light load to heavy
load.
Error Amplifier and Slope Compensation
inside NCP1595. No external circuitry is needed to stabilize
the device. The error amplifier provides an error signal to the
PWM comparator by comparing the feedback voltage
(800 mV) with internal voltage reference of 1.2 V.
cycles over 50%. A slope compensation circuit is provided
inside NCP1595 to overcome the potential instability. Slope
compensation consists of a ramp signal generated by the
synchronization block and adding this to the inductor
current signal. The summed signal is then applied to the
PWM comparator.
NCP1595 operates as a current mode buck converter with
After a minimum dead time, the N−Channel synchronized
A fully internal compensated error amplifier is provided
Current mode converter can exhibit instability at duty
1. PWM comparator output trips as the peak inductor
2. The inductor current has reached the current limit.
3. Overvoltage at output occurs.
current signal reaches a threshold level established
by the error amplifier.
DETAILED OPERATING DESCRIPTION
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Soft−Start and Current Limit
in−rush current during startup. This helps to reduce the
output voltage overshoot.
excess of 2 A. The intended output current of the system is
1.5 A. The ripple current is calculated to be approximately
350 mA with a 3.3 mH inductor. Therefore, the peak current
at 1.5 A output will be approximately 1.7 A. A 2 A set point
will allow for transient currents during load step. The current
limit circuit is implemented as a cycle−by−cycle current
limit. Each on−cycle is treated as a separate situation.
Current limiting is implemented by monitoring the
P−Channel switch current buildup during conduction with a
current limit comparator. The output of the current limit
comparator resets the PWM latch, immediately terminating
the current cycle.
Over−Voltage Protection
5% of its regulated voltage. In this case, the P−Channel main
switch will be reset and the N−Channel synchronized switch
is turn on to sink current from the output voltage which helps
to drop its feedback voltage back to the regulated voltage.
Thermal Shutdown
protect the integrated circuit in the event when maximum
junction temperature is exceeded. When activated, typically
at 160°C, the shutdown signal will disable the P−Channel
and N−Channel switch. The thermal shutdown circuit is
designed with 30°C of hysteresis. This means that the
switching will not start until the die temperature drops by
this amount. This feature is provided to prevent catastrophic
failures from accidental device overheating. It is not
intended as a substitute for proper heat sinking.
NCP1595 is contained in the thermally enhanced
DFN package.
A soft start circuit is internally implemented to reduce the
The current limit is set to allow peak switch current in
Overvoltage occurs when the feedback voltage exceeds
Internal Thermal Shutdown circuitry is provided to