TPS51219_15 Datasheet, PDF (18/39 Pages) Texas Instruments – High Performance, Single-Synchronous Step-Down Controller with Differential Voltage Feedback

TPS51219
SLUSAG1B – MARCH 2011 – REVISED OCTOBER 2011
D-CAP2™ Mode Operation
Figure 27 shows simplified model of D-CAP2™ architecture.
VSNS
4
C1
COMP
5
REFIN
2
R1 ^VREF
1
R2
C
C1
C
C2
R
C2
R
C1
G
+
+ _{2.0 V}
+_–
+
PWM
Comparator
www.ti.com
Control
Logic
and
Driver
V
IN
SW
12
DH
11
DL
10
L
X
V
OUT
ESR
C
OUT
R
LOAD
TPS51219
UDG-11011
Figure 27. Simplified Modulator Using D-CAP2™ Mode
When the TPS51219 operates in D-CAP2™ mode, connect the COMP and VSNS pins as shown in Figure 27.
The transconductance amplifier and the capacitance C1 configures the integrator. The D-CAP2™ mode in the
TPS51219 includes an internal feedback network enabling the use of very low ESR　output capacitor(s) such as
multi-layer ceramic capacitors (MLCC). The role of the internal network is to sense the ripple　component of the
inductor current information and then combine it with the voltage feedback signal.
Using R_C1=R_C2≡R_C and C_C1=C_C2≡C_C, 0-dB frequency of the D-CAP2™ mode is given by Equation 4. f₀ is
recommended to be lower than 1/3 of the switching frequency to secure proper phase margin. The integrator
time constant should be long enough compared to f₀, for example one decade low, as described in Equation 5.
f₀
=
R_C ´ C_C
2p ´ G ´ L_X ´ C_OUT
£
f_SW
3
where
• G is gain of the amplifier which amplifies the ripple current information generated by the compensation
circuit
(4)
g_M
f₀
£
2p ´ C1 10
(5)
The typical G value is 0.25, and typical R_CC_C time constant values for 500 kHz and 670 kHz operation are 32 μs
and 23 μs, respectively.
For example, when f_SW = 500 kHz and L_X=0.45 μH, C_OUT should be larger than 272 μF. At the selection of
capacitor, pay attention to its characteristics. For MLCC use X5R or better dielectric and take into account
derating of the capacitance by both DC bias and AC bias. When derating by DC bias and AC bias are 80% and
50%, respectively, the effective derating is 40% because 0.8 x 0.5 = 0.4. The capacitance of specialty polymer
capacitors may change depending on the operating frequency. Consult capacitor manufacturers for specific
characteristics.
18
Submit Documentation Feedback
Copyright © 2011, Texas Instruments Incorporated