US3036
The soft start capacitor also provides a delay in the turn
on of the output which is given by:
Td=CSS*K
Where K=30 ms/uF
For example for CSS=0.1uF,
Td=0.1* 30=3 ms
APPLICATION INFORMATION
Introduction
The US3036 device is an application specific product
designed to provide an on board switching supply for the
new generation of microprocessors requiring separate
Core and I/O supplies where the load current demand
from the I/O supply requires this regulator to also be a
switching regulator such as the motherboard applica-
tions with AGP slot or the Pentium II with on board 5V to
3.3V converter. The US3036 provides an easy and low
cost switching regulator solution for Vcore and 3.3V
supplies with true short circuit protection.
Switcher Current Limit Protection
The US3034 uses an external current sensing resistor
and compares the voltage drop across it to a programmed
voltage which is set externally via a resistor (RcL) placed
between the “CS-” terminal of the IC and Vout. Once the
voltage across the sense resistor exceeds the thresh-
old, the soft start capacitor pulls up to 12V, pulling up
the inverting pin of the error comparator higher than non
inverting which causes the external MOSFET to shut
off. At this point the C.S comparator changes its state
and pulls the soft start capacitor to Vcc which is 12V
and shutting the PWM drive. After the output drive is
turned off, an internal 10uA current source slowly dis-
charge the soft start capacitor to approximately 5.7V,
before the output starts to turn back on causing a long
delay before the MOSFET turns back on. This delay
causes the catch diode to cool off between the current
limit cycles allowing the converter to survive a short cir-
cuit condition. An example is given below as how to
select the current limiting components. Assuming the
desired current limit point is set to be 20A and the cur-
rent sense resistor Rs=5mW, then the current limit pro-
gramming resistor,RcL is calculated as :
Vcs=IcL*Rs=20*0.005=0.1V
RcL=Vcs/Ib=(0.1V)/(20uA)=5kW
Where: Ib=20uA is the internal current source of the
US3034
The peak power dissipated in the C.S. resistor is :
Ppk=(IcL^2)*Rs=20^2*0.005=2W
However, the average power dissipated is much lower
than 2W due to the long off time caused by the hiccup
circuit of 3034. The average power is in fact the short
circuit period divided by the short circuit period plus the
off time or "hiccup" period. For example, if the short cir-
cuit lasts for TSC=100uSec before the 3034 enters hic-
cup, the average power is calculated as :
Switching Controller Operation
The operation of the switching controller is as follows :
after the power is applied, the output drive pin, "Drv" goes
low turning the P MOS to 100% duty cycle and the the
current in the inductor charges the output capacitor caus-
ing the output voltage to increase. When output reaches
a pre-programmed set point the feedback pin "Vfb" ex-
ceeds 1.25V causing the output drive to switch low and
the "Vhyst" pin to switch high which jumps the feed-
back pin higher than 1.25V resulting in a fixed output
ripple which is given by the following equation :
dVo=(Rt/Rh)x(Vcc -1)
Where:
Rt=Resistor connected from Vout to the Vfb pin of
US3036
Rh=Resistor connected from Vfb pin to Vhyst pin.
For example, if Rt=1k and Rh=200k, then the output
ripple is :
dVo=(1/200)x4=20 mV
The advantage of fixed output ripple method is that when
the output voltage changes from 2V to 3.5V, the ripple
voltage remains the same which is important in meeting
the Intel maximum tolerance specification.
Soft Start
The soft start capacitor must be selected such that dur-
ing the start up when the output capacitors are charging
up, the peak inductor current does not reach the current
limit treshold. A minimum of 0.1uF capacitor insures
this for most applications. During start up the soft start
capacitor is charged up to approximately 6V keeping
the output shutdown before an internal 10uA current
source start discharging the soft start capacitor which
slowly ramps up the inverting input of the PWM com-
parator, Vfb. This insures the output to ramp up at the
same rate as the soft start cap thereby limiting the input
current. For example, with 0.1uF and the 10uA internal
current source the ramp up rate is (DV/ Dt)=I/Css = 10/
0.1=100V/Sec or 0.1V/mSec. Assuming that the output
capacitance is 6000uF, the peak input current will be:
Iin(pk)=Css*(DV/ Dt)=6000uF*(0.1V/mSec)=0.6A
Pave=Ppk*DSC
Where:
DSC=TSC/THCP
THCP=CSS*M
Where M=630 ms/uF & CSS, is the soft start capacitor
For example for CSS=0.1uF & TSC=100uSec=0.1mS
THCP=0.1* 630=63 ms
Pave=2*(0.1/63)=3.2 mW
Without "hiccup" technique, the power dissipation
of the resistor is 2W.
Rev. 1.1
12/4/98
4-6
UNISEM [ UNISEM ]
