MAX1870AETJ+ Maxim Integrated Products, MAX1870AETJ+ Datasheet - Page 24

• Manufacturer Part Number: MAX1870AETJ+
• Description: Battery Management Li+ Step Up/Step Down Battery Charger
• Manufacturer: Maxim Integrated Products

Specifications of MAX1870AETJ+

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Table 3. Constant Voltage Loop Poles and Zeros
Step-Up/Step-Down
Li+ Battery Charger
Setting the LTF = 1 to solve for the unity-gain frequency
yields:
For stability, choose a crossover frequency lower than
1/10th of the switching frequency. The crossover fre-
quency must also be below the RHP zero, calculated at
maximum charge current, minimum input voltage, and
maximum battery voltage.
Choosing a crossover frequency of 13kHz and solving for
R
MODE = V
C
V
24
BATT
CV
OUT
NO.
1
2
3
4
5
using the component values listed in Figure 1 yields:
______________________________________________________________________________________
= 16.8V
= 22µF
f
CO CV
_
CC
LTF
CCV Zero
RHP Zero
CCV Pole
Output
Output
NAME
Zero
Pole
=
(4 cells)
=
GM
GM
PWM
PWM
x G
f
x
P CV
f
f
Z OUT
f
P OUT
_
Z CV
sC
MV
_
_
f
=
_
R
RHPZ
GMV = 0.1µA/mV
GM
f
CO_CV
CV
OUT
2
⎛
⎜
⎝
=
CALCULATION
π
2π
PWM
=
=
=
2π
=
x L I
G
R
2π
2π
x C
2π
MV
2
CV
= 13kHz
x R
V
π
= 1.85A/V
IN
OUT
x R
x R
OUT
x R C
V
x L I
2
OGMV
IN
1
1
CV
1
ESR
⎞
⎟
⎠
L
1
V
OUT
L
C
OUT
C
C
CV
OUT
CV
Lowest Frequency Pole created by C
resistance. Since R
a low-frequency pole.
Voltage-Loop Compensation Zero. If this zero is lower than the
output pole, f
approximates a single-pole response near the crossover
frequency. Choose C
below crossover to ensure adequate phase margin.
Outp ut P ol e For m ed w i th the E ffecti ve Load Resi stance R
Outp ut C ap aci tance C
affect the stab i l i ty of the system or the cr ossover fr eq uency.
Output ESR Zero. This zero can keep the loop from crossing
unity gain if f
frequency. Therefore, choose a capacitor with an ESR zero
greater than the crossover frequency.
S tep - U p M od e RH P Z er o. Thi s zer o occur s b ecause of the i ni ti al
op p osi ng r esp onse of a step - up conver ter . E ffor ts to i ncr ease the
i nd uctor cur r ent r esul t i n an i m m ed i ate d ecr ease i n cur r ent
d el i ver ed , al thoug h eventual l y r esul t i n an i ncr ease i n cur r ent
d el i ver ed . Thi s zer o i s d ep end ent on char g e cur r ent and m ay
cause the system to g o unstab l e at hi g h cur r ents w hen i n step - up
m od e. A r i g ht- hal f- p l ane zer o i s d etr i m ental to b oth p hase and
g ai n. To ensur e stab i l i ty und er m axi m um l oad i n step - up m od e,
the cr ossover fr eq uency m ust b e l ow er than hal f of f
R
To ensure that the compensation zero adequately can-
cels the output pole, select f
Figure 10 shows the Bode Plot of the voltage-loop fre-
quency response using the values calculated above.
When the MAX1870A regulates the charge current or the
wall adapter current, the system stability does not
depend on the output capacitance. The simplified
schematic in Figure 11 describes the operation of the
MAX1870A when the charge-current loop (CCI) is in con-
trol. The simplified schematic in Figure 12 describes the
operation of the MAX1870A when the source-current
L
= 0.2Ω
Z_OUT
P_OUT
R
Charge-Current and Wall-Adapter-Current
CV
OGMV
, then the loop transfer function
C
is less than the desired crossover
=
CV
CV
OU T
2
π
to place this zero at least 1 decade
DESCRIPTION
≥ (R
. R
GMV x GM
is very large (R
C
x C
L
CV
i nfl uences the D C g ai n b ut d oes not
L
OUT
/ R
≥ 440pF
Z_CV ≤
CV
x f
f
OSC
CV
) x C
PWM
CO CV
and GMV’s finite output
Loop Compensation
OGMV
_
f
= 400kHz
P_OUT
OUT
=
> 10MΩ), this is
10
R H P Z
.
k
L
Ω
.
and the