Datasheet

Booststarts
LDOoff
VoutnIsw
lim
= 120 mA
VoutpIsw
lim
= 220 mA
LDO = 100 mA currentlimit
VoutnIsw
lim
= 120 mA
VoutpIsw
lim
= fullcurrentlimit
LDO = fullcurrentlimit
VoutnIsw
lim
= fullcurrentlimit
Voutn= -0.4 V
Voutp= 3 V
Voutn= -1 V
TPS65137
SLVS929A –MAY 2010–REVISED OCTOBER 2012 www.ti.com
DETAILED DESCRIPTION
The TPS65137 consists of a boost converter using a LDO as post regulator. The output voltage of the boost
converter is regulated to operate the internal LDO above its dropout voltage maintaining best line and load
regulation of OUTP. The internal LDO disconnects OUTP during shutdown and allows regulation of the output
when the input voltage is higher than OUTP. The LDO minimizes the output voltage ripple of OUTP. The
negative output uses a buck boost converter topology operating in DCM (Discontinuous Conduction Mode)
providing superior line regulation. In order to adjust the output voltage of the negative converter a digital interface
can be used to program the output voltage. To achieve high efficiency over the entire load current range the
device reduces the switching frequency with the load current using its internal voltage controlled oscillator (VCO).
Since the boost converter output CB is post regulated by the integrated LDO (Low Dropout Regulator) the output
voltage ripple is minimized and the line transient response is at its best. Because of this topology the operation
mode of the boost converter has minimum effect on the output voltage ripple observed on OUTP. The boost
converter, as well as the negative converter operate in peak current mode using the VCO (Voltage Controlled
Oscillator) while operating in DCM (Discontinuous Conduction Mode). When entering CCM (Continuous
Conduction Mode) the converter operates in peak current control using fixed off time control.
POWER SAVE MODE OPERATION
In order to maintain high efficiency over the entire load current range the converter reduces its switching
frequency as the load current decreases. To maintain a controlled switching frequency a voltage controlled
oscillator (VCO) is used.
SOFT START AND SHORT CIRCUIT PROTECTION
The device has a soft-start implemented limiting inrush current during turn on. The device is also protected
against short circuits of the outputs to ground or when the outputs shorted together. This is implemented with two
output voltage thresholds determining the device switch current limit and LDO operation shown in Figure 14.
Figure 14. Soft Start and Short Circuit Thresholds
When the device is enabled pulling CTRL pin high then the boost converter and buck converter starts with
reduced switch current limit. During this period of time the LDO is turned off. As V
NEG
reaches –0.4V then the
LDO is turned on having a 100mA current limit. The switch current limit of both outputs is increased to 220mA
and 120mA. When V
POS
reaches 3V and V
NEG
reaches –1V, then both outputs operate with full current limit. This
architecture limits the inrush current during start-up and protects the device during short circuits events. When
the positive output is shorted to the negative output then the device cycles between the first and second section
of the start-up sequence. By that, the output current cycles between zero and 100mA. This protects the device
and avoids excessive power dissipation during short circuit conditions. With this architecture the device is able to
start-into full load current once V
POS
exceeds 3V and V
NEG
is lower than –1V.
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