MAX1535A Datasheet, PDF (29/39 Pages) Maxim Integrated Products – Highly Integrated Level 2 SMBus Battery Charger

Highly Integrated Level 2 SMBus
Battery Charger
Additionally, R_CV is much higher impedance than C_CV
and dominates the series combination of R_CV and C_CV,
so:
R_OGMV × (1+ SC_CV × R_CV )
(1+ SC_CV × R_OGMV )
≅
R_CV
C_OUT is also much lower impedance than R_L near
crossover so the parallel impedance is mostly capaci-
tive and:
R_L
≅¹
(1+ SC_OUT × R_L) SC_OUT
If R_ESR is small enough, its associated output zero has
a negligible effect near crossover and the loop-transfer
function can be simplified as follows:
LTF
=
GM_OUT
×
R_CV
SC_OUT
GMV
Setting the LTF = 1 to solve for the unity-gain frequency
yields:
f_{CO _ CV}
=
GM_OUT
× GMV
×
2π
R_CV
× C_OUT
For stability, choose a crossover frequency lower than
1/10th of the switching frequency. For example, choose
a crossover frequency of 30kHz and solving for R_CV
using the component values listed in Figure 1 yields
R_CV = 20kΩ:
R_CV
=
2π × C_OUT × f_{CO_ CV}
GMV × GM_OUT
= 10kΩ
V_BATT = 16.8V
GMV = 0.125µA/mV
I_CHG = 4A
GM_OUT = 5A/V
C_OUT = 2 × 22µF
f_OSC = 400kHz
R_L = 0.2Ω
f_{CO_CV} = 30kHz
CSI
IMAX
2V
CCMP
LVC
IMIN
100mV
50mV
DCIN
BATT
ZCMP
OFF-TIME
COMPUTE
R
Q
R
Q
OFF-TIME
ONE-SHOT
Figure 9. DC-to-DC Converter Block Diagram
TO
DH
DRIVER
TO
DL
DRIVER
GM_OUT
BATT
R_ESR
R_L
CCV
C_OUT
GMV
R_CV
R_OGMV
C_CV
REF
Figure 10. CCV Loop Diagram
To ensure that the compensation zero adequately can-
cels the output pole, select f_{Z_CV} ≤ f_{P_OUT}:
C_CV ≥ (R_L/R_CV) C_OUT
C_CV ≥ 4nF (assuming 4 cells and 4A maximum charge
current)
Figure 11 shows the Bode plot of the voltage-loop fre-
quency response using the values calculated above.
______________________________________________________________________________________ 29