Datasheet

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Loop Compensation and External Parts Selection

Current Mode Operation

TPS51120

SLUS670B – JULY 2005 – REVISED FEBRUARY 2007

DETAILED DESCRIPTION (continued)

Table 5. Protection States (VOUT1 = 5 V, VOUT2 = 3.3 V)

DRVH1 DRVL1 DRVH2 DRVL2 PGOOD1 VREG5 VREG3 VREF2 FOR

PGOOD2 RESTART

UVPch1 Low Low Low Low Low/Low LDO 5 V LDO 3.3 V On Toggle EN1

UVPch2 Low Low Low Low Low/Low LDO 5 V LDO 3.3 V On Toggle EN2

OVPch1 Low High Low Low Low/Low LDO 5 V LDO 3.3 V On Toggle EN1

OVPch1 Low Low Low High Low/Low LDO 5 V LDO 3.3 V On Toggle EN2

Thermal SHDN Low Low Low Low Low/Low Off Off Off Lower Package

Temperature

VIN < 5.0 Normal Normal Normal Normal Low/Normal SW 5 V SW 3.3 V On Raise VIN

VREG Low Low Low Low Low/Low LDO but LDO 3.3 V On Raise VIN,

UVLO dropping Reduce 5V

current

OCPch1 Limited Estended Normal Normal Low/Normal LDO 5 V SW 3.3 V On Reduce CH1

Duty Duty Current

OCPch2 Normal Normal Limited Estended Normal/Low SW 5 V LDO 3.3 V On Reduce CH2

Duty Duty Current

EN1 Low Low Low Normal Normal Low/Normal LDO 5 V SW 3.3 V On Float or tie to

VREG5

EN2 Low Normal Normal Low Low Normal/Low SW 5 V LDO 3.3 V On Float or tie to

VREG5

EN1, EN2, EN3 Low Low Low Low Low/Low LDO 5 V Off Off Float EN3, then

Low float EN1,2 or

tie to VREG5

EN5, EN1 Low Low Low Low Low Low/Low Off LDO 3.3 V Off Float EN5 or tie

to VBAT, tie

EN1 to VREG5

A buck converter using TPS51120 current mode operation can be partitioned into three portions, a voltage

divider, an error amplifier and a switching modulator. By linearizing the s witching modulator, we can derive the

transfer function of the whole system. Since current mode scheme directly controls the inductor current, the

modulator can be linearized as shown in Figure 1 .

Figure 1. Linearizing the Modulator

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