TNY375-380
reduced, and the clock frequency is reduced to half that at the
highest current limit level (Figure 8). At very light loads, the
current limit will be reduced even further (Figure 9). Only a small
percentage of cycles will occur to satisfy the power consumption
of the power supply. The response time of the ON/OFF control
scheme is very fast compared to PWM control. This provides
tight regulation and excellent transient response.
With the TNY37ꢀ and TNY376, no bias winding is needed to
provide power to the chip because it draws the power directly
from the DRAIN pin (see Functional Description above). This
eliminates the cost of a bias winding and associated components.
For the TNY377-380 or for applications that require very low
no-load power consumption (ꢀ0 mW), a resistor from a bias
winding to the BYPASS/MULTI-FUNCTION pin can provide the
power to the chip. The minimum recommended current supplied
is IS2 + IDIS. The BYPASS/MULTI-FUNCTION pin in this case will
be clamped at 6.4 V. This method will eliminate the power draw
from the DRAIN pin, thereby reducing the no-load power
consumption and improving full-load efficiency.
Power Up/Down
The TinySwitch-PK requires only a 0.1 mF capacitor on the
BYPASS/MULTI-FUNCTION pin to operate with standard
current limit. Because of its small size, the time to charge this
capacitor is kept to an absolute minimum, typically 0.6 ms. The
time to charge will vary in proportion to the BYPASS/MULTI-
FUNCTION pin capacitor value when selecting different current
limits. Due to the high bandwidth of the ON/OFF feedback,
there is no overshoot at the power supply output. When an
external resistor (4 MW) is connected from the power supply
positive DC input to the EN/UV pin, the power MOSFET
switching will be delayed during power-up until the DC line
voltage exceeds the threshold (100 V). Figures 10 and 11 show
the power-up timing waveform in applications with and without
an external resistor (4 MW) connected to the EN/UV pin.
Current Limit Operation
Each switching cycle is terminated when the DRAIN current
reaches the current limit of the device. Current limit operation
provides good line ripple rejection and relatively constant power
delivery independent of input voltage.
BYPASS/MULTI-FUNCTION Pin Capacitor
The BYPASS/MULTI-FUNCTION pin can use a ceramic
capacitor as small as 0.1 mF for decoupling the internal power
supply of the device. A larger capacitor size can be used to
adjust the current limit. A 1 mF BP/M pin capacitor will select a
lower current limit equal to the standard current limit of the next
smaller device, and a 10 mF BP/M pin capacitor will select a
higher current limit equal to the standard current limit of the next
larger device. The TNY37ꢀ and TNY376 MOSFETs do not have
the capability to match the current limit of the next larger
devices in the family. The current limit is therefore increased to
the maximum capability of their respective MOSFETs. The
higher current limit level of the TNY380 is set to 110ꢀ mA typical.
The smaller current limit of the TNY37ꢀ is set to 32ꢀ mA.
During power-down, when an external resistor is used, the
power MOSFET will switch for 32 ms after the output loses
regulation. The power MOSFET will then remain off without any
glitches since the undervoltage function prohibits restart when
the line voltage is low.
Figure 12 illustrates a typical power-down timing waveform.
Figure 13 illustrates a very slow power-down timing waveform,
as in standby applications. The external resistor (4 MW) is
connected to the EN/UV pin in this case to prevent unwanted
restarts.
7
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Rev. C 09/12
POWERINT [ Power Integrations ]
