Datasheet

PRI
HS LS
V - 0.829V
R = R ×
0.829V
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TPS55010
www.ti.com SLVSAV0A –APRIL 2011–REVISED JUNE 2011
Detailed Description
The TPS55010 is a half bridge transformer driver designed to implement a high efficiency, low power isolated
supply. The primary side feedback implemented using two resistors and a primary side capacitor provides
excellent regulation over line and load compared to an open loop push pull converter.
The half bridge power stage consists of two integrated n-channel MOSFETs with 45 mΩ on resistance. The drive
voltage for the integrated high side MOSFET is supplied by a capacitor between the BOOT and PH pins. The
switching frequency is adjusted using a resistor to ground on the RT/CLK pin. The device has an internal phase
lock loop (PLL) on the RT/CLK pin that is used to synchronize the high side power switch turn on to a falling
edge of an external system clock. The wide switching frequency of 100 kHz to 2000 kHz (300kHz to 2000kHz in
CLK mode) allows for efficiency, size optimization or noise avoidance when selecting the switching frequency.
The TPS55010 has a typical default start up voltage of 2.6 V. The EN pin has an internal pull-up current source
that can be used to adjust the input voltage under voltage lockout (UVLO) with two external resistors. In addition,
the pull up current provides a default condition when the EN pin is floating for the device to operate. The total
operating current for the TPS55010 is typically 360 µA when not switching and under no load. When the device
is disabled, the supply current is less than 5 µA. The slow start (SS) pin is used to minimize inrush currents
during start up.
Fixed Frequency PWM Control
The TPS55010 uses an adjustable fixed frequency, peak current mode control. The primary voltage is compared
through external resistors on the VSENSE pin to an internal voltage reference by an error amplifier which drives
the COMP pin. An internal oscillator initiates the turn on of the high side power switch. The error amplifier output
is compared to the high side power switch current. When the power switch current reaches the COMP voltage
level the high side power switch is turned off and the low side power switch is turned on. The COMP pin voltage
increases and decreases as the output current increases and decreases. The device implements a current limit
by clamping the COMP pin voltage to a maximum level. The TPS55010 adds a compensating ramp to the switch
current signal. This slope compensation prevents sub-harmonic oscillations as duty cycle increases.
Half Bridge and Bootstrap Voltage
The TPS55010 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT and
PH pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be
0.1 µF. A ceramic capacitor with an X7R or X5R grade dielectric and a voltage rating of 10 V or higher is
recommended because of the stable characteristics over temperature and voltage.
Error Amplifier
The TPS55010 uses a transconductance error amplifier. The amplifier compares the VSENSE voltage to the
lower of the SS pin voltage or the internal 0.829 V voltage reference. The transconductance of the error amplifier
is 245 µA/V. The frequency compensation components are placed between the COMP pin and ground.
Voltage Reference
The voltage reference system produces a precise ±3.0% voltage reference over temperature by scaling the
output of a temperature-stable band gap circuit. The band gap and scaling circuits produce 0.829 V at the non-
inverting input of the error amplifier.
Adjusting the Output Voltage
The primary side voltage is set with a resistor divider from the primary side capacitor to the VSENSE pin. It is
recommended to use 1% tolerance or better divider resistors. Start with a 10 kΩ for the R
LS
resistor and use
Equation 1 to calculate R
HS
. The output voltage is a function of the primary voltage, transformer turns ratio and
forward voltage of the diode.
(1)
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