Datasheet

 
 
SLVS485 − AUGUST 2003
www.ti.com
10
Figure 18
TPS64200
STARTUP TIMING
EN
V
O
I
I
I
(Inductor)
V
I
= 3.8 V,
V
O
= 3.3 V,
R
L
= 1.66 ,
T
A
= 25°C
100 ms/Div
DETAILED DESCRIPTION
Operation
The TPS6420x is a nonsynchronous step-down controller which is operating with a minimum on-time/minimum
off-time control. An external PMOS is turned on until the output voltage reaches its nominal value or the current
limit is exceeded. If the current limit is exceeded, the PMOS is switched off and stays off for the minimum
off-time. After that the PMOS is switched on again. When the nominal output voltage is reached, the PMOS
is switched off and stays off until the output voltage dropped below its nominal value.
Operating Modes
When delivering low or medium output current, the TPS6420x operate in discontinuous mode. With every
switching cycle, the current in the inductor starts at zero, rises to a maximum value and ramps down to zero
again. As soon as the current in the inductor drops to zero, ringing occurs at the resonant frequency of the
inductor and stray capacitance, due to residual energy in the inductor when the diode turns off. Ringing in
discontinuous mode is normal and does not have any influence on efficiency. The ringing does not contain much
energy and can easily be damped by an RC snubber. See the application section for further details.
With high output current, the TPS6420x operate in continuous current mode. In this mode, the inductor current
does not drop to zero within one switching cycle. The output voltage in continuous mode is directly dependant
on the duty cycle of the switch.
Variable Minimum On-Time (TPS64201 to TPS64202 Only)
The minimum on-time of the device is 1.6 µs. At light loads, this would cause a low switching frequency in the
audible range because the energy transferred to the output during the on-time would cause a higher rise in the
output voltage than needed and therefore lead to a long off−time until the output voltage dropped again. To avoid
a switching frequency in the audible range the TPS64201 and TPS64202 can internally reduce the minimum
on time in three steps from 1.6 µs to 800 ns, 400 ns and 200 ns. The on-time is reduced by one step if the
switching frequency dropped to a lower value than 50 kHz. This keeps the frequency above the audio frequency
over a wide load range and also keeps the output voltage ripple low.