NCP1579
http://onsemi.com
6
DETAILED OPERATING DESCRIPTION
General
The NCP1579 is a PWM controller intended for DC−DC
conversion from 5.0 V & 12 V buses. The devices have a 1
A internal gate driver circuit designed to drive N−channel
MOSFETs in a synchronous−rectifier buck topology. The
output voltage of the converter can be precisely regulated
down to 800 mV ±2.0% when the V
FB
pin is tied to V
OUT
.
The switching frequency, is internally set to 275 kHz. A high
gain operational transconductance error amplifier (OTA) is
used.
Duty Cycle and Maximum Pulse Width Limits
In steady state DC operation, the duty cycle will stabilize
at an operating point defined by the ratio of the input to the
output voltage. The devices can achieve an 80% duty cycle.
There is a built in off−time which ensures that the bootstrap
supply is charged every cycle. Both parts can allow a 12 V
to 0.8 V conversion at 275 kHz.
Input Voltage Range (V
CC
and BST)
The input voltage range for both V
CC
and BST is 4.5 V to
13.2 V with respect to GND and PHASE, respectively.
Although BST is rated at 13.2 V with respect to PHASE, it
can also tolerate 26.4 V with respect to GND.
External Enable/Disable
When the Comp pin voltage falls or is pulled externally
below the 400 mV threshold, it disables the PWM Logic and
the gate drive outputs. In this disabled mode, the operational
transconductance amplifier (EOTA) output source current is
reduced and limited to the Soft−Start mode of 10 mA.
Normal Shutdown Behavior
Normal shutdown occurs when the IC stops switching
because the input supply reaches UVLO threshold. In this
case, switching stops, the internal SS is discharged, and all
GATE pins go low. The switch node enters a high impedance
state and the output capacitors discharge through the load
with no ringing on the output voltage.
External Soft−Start
The NCP1579 features an external soft−start function,
which reduces inrush current and overshoot of the output
voltage. Soft−start is achieved by using the internal current
source of 10 mA (typ), which charges the external integrator
capacitor of the transconductance amplifier. Figure 8 is a
typical soft−start sequence. This sequence begins once V
CC
surpasses its UVLO threshold and OCP programming is
complete. During soft−start, as the Comp Pin rises through
400 mV, the PWM Logic and gate drives are enabled. When
the feedback voltage crosses 800 mV, the EOTA will be
given control to switch to its higher regulation mode output
current of 120 mA.
Figure 8. Soft−Start Implementation
4.0 V
0.85 V
0.8 V
Comp
V
CC
V
fb
BG
TG
V
out
NORMALSSPOR
UVLO
550 mV
50 mV
OCP
Program
ming
