Datasheet
SLUS489 − OCTOBER 2001
13
www.ti.com
APPLICATION INFORMATION
start-up and soft-start
The TPS43000 incorporates an UVLO circuit that disables the output drivers when the voltage at the VIN pin
is below 1.65 V. In order to prevent the converter from oscillating during low input voltage startup, the UVLO
circuit is designed with 200 mV of hysteresis and the converter remains on until VIN drops below 1.45 V.
The TPS43000 has a built-in soft-start that varies as a function of the switching frequency. The soft-start is a
closed-loop soft-start, meaning that the reference input to the error amplifier is ramped up over the soft-start
interval and the converter control loop is allowed to track the ramping reference signal. The soft-start interval
is set to approximately 2000 oscillator clock cycles. This method generally allows for faster soft-start times with
minimal output voltage overshoot at startup. During start-up, the synchronous rectifier is held off until the COMP
pin reaches 700 mV.
programming the PWM frequency
The oscillator frequency is programmed by a resistor from the RT pin to ground. The approximate operating
frequency is determined by the equation:
f
SW
(MHz) +
38
R
T
(
kW
)
The maximum operating frequency is 2 MHz. Some applications may want to remain in a fixed-frequency mode
of operation, even at light load, rather than going into PFM mode. This lowers efficiency at light load. One way
to improve the efficiency while maintaining fixed frequency operation is to lower the PWM frequency under
light-load conditions. This can be easily done, as shown in Figure 1. By adding a second timing resistor and a
small MOSFET switch, the host can switch between two discrete frequencies at any time.
UDG−01035
1
3
2
4
5
6
7
8
SYNC/SD
CCS
RT
CCM
BUCK
COMP
FB
TPS43000
9
10
11
12
13
14
15
16
VIN
VOUT
NDRV
GND
PDRV
VP
SWP
SWN
RT1
2N7002
RT2
FREQUENCY
CONTROL
R1
R2
PFM
V
OUT
Figure 1. Changing the PWM Frequency
(5)