US3034
PIN DESCRIPTIONS
PIN # PIN SYMBOL PIN DESCRIPTION
A resistor divider from this pin to the output of the switching regulator and ground sets the
Core supply voltage.
3
VFB
This pin is connected to the minus side of the external current sense resistor. An internal
current source together with an external resistor in series with this pin programs the
current limit threshold voltage. This voltage divided by the external current sense resistor
sets the current limit threshold.
6
C.S-
This pin is connected to the plus side of the external current sense resistor. A resistor in
series with this pin and a capacitor connected between this pin and pin 6 provides a high
frequency filtering for the noise spikes of turn on and turn off switching.
This pin is connected to the IC substrate and must be connected to the lowest potential
in the system.
The PWM output of the switching controller. This pin is a totem pole drive that is con-
nected to the gate of the power MOSFET. A resistor may be placed from this pin to the
gate in order to reduce switching noise.
7
C.S+
5
1
Gnd
Drv
A resistor and a 10pF capacitor is connected from this pin to the VFB1 pin to set the
output ripple voltage for the switching regulator.
4
2
VHYST
V12
This pin supplies the voltage to the PWM drive and hysterises circuitry and it is con-
nected to the 12V supply. A 1 uF, high frequency capacitor must be connected from this
pin to ground to provide the peak current for charging and discharging of the MOSFET.
This pin provides the soft start for the regulator during power up. It also sets a long off
time when the converter goes into current limiting, providing low duty cycle for the catch
diode allowing it to survive during short circuit.
8
S.S
BLOCK DIAGRAM
V12
2
Vhyst
4
Vfb
3
5V Reg
R
UVLO
20uA
CS-
6
Drv
1
PWM Control
1.25V
3R
Vref
CS+
7
Gnd
5
S.S. / Hiccup
Control
SS
8
3034blk1-1.1
Figure 1 - Simplified block diagram of the US3034
Rev. 1.8
1/13/99
4-3
UNISEM [ UNISEM ]
