US3036

4-6

Rev. 1.1

12/4/98

APPLICATION INFORMATION

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.

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 (

∆V/ ∆t)=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*(

∆V/ ∆t)=6000uF*(0.1V/mSec)=0.6A

The soft start capacitor also provides a delay in the turn

on of the output which is given by:

Where K=30 ms/uF

Td=0.1* 30=3 ms

Switcher Current Limit Protection

The US3034 uses an external current sensing resistor

and compares the voltage drop across it to a programmed

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-

Ω, then the current limit pro-

Ω

US3034

The peak power dissipated in the C.S. resistor is :

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-

cup, the average power is calculated as :

Where:

Pave=2*(0.1/63)=3.2 mW

Without "hiccup" technique, the power dissipation

of the resistor is 2W.