APW7079
Function Description
Control Scheme
The converter monitors the output voltage. When the in-
ternal feedback voltage falls below the reference voltage,
the main switch turns on and the inductor current ramps
up. The main switch turns off when the current reaches
the peak current limit of typical 600mA. The second crite-
rion that turns off the switch is the maximum on-time of
4ms (typical). As the main switch is turned off, the syn-
chronous switch is turned on and delivers the current to
the output. The main switch remains off for a minimum of
900ns (typical), or until the internal feedback voltage drops
below the reference voltage. By the control scheme with
low quiescent current of 11mA (typical), the converter gets
high efficiency over a wide load range.
Start-Up
A startup oscillator circuit is integrated in the APW7079.
When the power is applied to the device, the circuit pumps
the output voltage high. Once the output voltage reaches
1.4V (typ), the main DC-DC circuitry turns on and boosts
the output voltage to the final regulation point.
Synchronous Rectification
The internal synchronous rectifier eliminates the need
for an external Schottky diode, thus reducing cost and
board space. During the cycle off-time, the P-channel
MOSFET turns on and shunts the MOSFET body diode.
As a rewsult, the synchronous rectifier significantly im-
proves efficiency without the addition of an external
component. Conversion efficiency can be as high as 92%.
Over-Temperature Protection
The over-temperature circuit limits the junction tempera-
ture of the APW7079. When the junction temperature ex-
ceeds 150°C, a thermal sensor turns off the power
MOSFETs, allowing the devices to cool. The thermal sen-
sor allows the converter to start a start-up process and
regulate the output voltage again after the junction tem-
perature cools by 40°C.The OTP is designed with a 40°C
hysteresis to lower the average TJ during continuous ther-
mal overload conditions, increasing lifetime of the device.
Copyright ã ANPEC Electronics Corp.
8
www.anpec.com.tw
Rev. A.3 - Oct., 2008
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
