TPS51511_15 Datasheet, PDF (16/30 Pages) Texas Instruments – Synchronous Switcher Controller With 2-A LDO for GPU Power

TPS51511
SLVS735A – FEBRUARY 2007 – REVISED APRIL 2007
www.ti.com
ENLDO
Hi
ENSW
X
Lo
Hi
Lo
Lo
Table 1. OD and ODOFF Functions
ODOFF ⁽¹⁾
> 1.25 V
0.716 V < ODOFF < 1.25 V and lasts longer than 45 µs
0.716 V < ODOFF < 1.25 V and lasts less than 45 µs
0.678 V < ODOFF < 0.716 V
< 0.678 V
> 1.25 V
< 1.25 V
X
OD ⁽²⁾
Off
Off
Keep State
Keep State
On
Off
On
On
Applications ⁽³⁾
D
D&G
G
G
D&G
D
D&G
G
(1) All voltages are typical values.
(2) "Keep state" means no state change with previous state, “On” means open drain MOSFET is turned on and OD is connected to ground
through this MOSFET. “Off” means open drain MOSFET off.
(3) “D” stands for dynamic voltage change “G” stands for LDO PGOOD .
Advanced PGOOD Mask
The advanced PGOOD mask feature allows the switcher to dynamically change the output voltage while
maintaining the PGOOD and OVP functions during the transition.
During the dynamic voltage change, if the output voltage is lower than 1.36 V (typ) the high-end PGOOD
comparator and OVP comparator is masked. If the output voltage is set equal to or higher than 1.36 V (typ), the
blanking circuit is disabled, and the high-end PGOOD threshold is set to 110%, and the OVP threshold is set to
115% of the output-voltage setting . When dynamic voltage change is implemented, a 1-nF ceramic capacitor
must be added in parallel with the top resistor of the voltage divider as shown in Figure 27.
APPLICATION INFORMATION
LOOP COMPENSATION AND EXTERNAL PARTS SELECTION
Current Mode Operation
A buck converter using TPS51511 current mode operation can be partitioned into three portions, a voltage
divider, an error amplifier and a switching modulator. By linearizing the switching modulator, we can derive the
transfer function of the whole system. Since the current-mode scheme directly controls the inductor current, the
modulator can be linearized as shown in Figure 25.
Voltage Divider
R1
R2
Cc2
Gm = 300 mS
Filter
0.75 V
Ref
PWM
COMP
Rc
Ro = 75 M
Cc
VOSW
Control
Logic
and
Driver
DRVH
DRVL
V_IN
Lx
CS
R_S
PGND
ESR
R_L
C_O
Error Amplifier
Switching Modulator
Figure 25. Simplified Current-Mode Functional Blocks
In this representation, the inductor is located inside the local feedback loop, and its inductance does not appear
in the small-signal model. As a result, a modulated current source including the power inductor can be modeled
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