TPS64200,TPS64201

TPS64202,TPS64203

SLVS485 − AUGUST 2003

www.ti.com

10

Figure 18

TPS64200

STARTUP TIMING

EN

VO

II

I(Inductor)

VI = 3.8 V,

VO = 3.3 V,

RL = 1.66 Ω,

TA = 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.