MAX1870AETJ+ Maxim Integrated Products, MAX1870AETJ+ Datasheet - Page 23

• 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

The MAX1870A includes input and output-voltage feed-
forward to maintain pseudo-fixed-frequency (400kHz)
operation. The time in state B is set according to the
input voltage, output voltage, and a time constant. In
step-up/step-down mode the switching frequency is
effectively cut in half to allow for both the step-up cycle
and the step-down cycle. The switching frequency is
typically between 350kHz and 405kHz for V
8V and 28V. See the Typical Operating Characteristics.
Each of the three regulation loops (the battery voltage,
the charge current, and the input current limit) are com-
pensated separately using the CCV, CCI, and CCS
pins, respectively. Compensate the voltage regulation
loop with a 10kΩ resistor in series with a 0.01µF capaci-
tor from CCV to GND. Compensate the charge current
loop and source current loop with 0.01µF capacitors
from CCI to GND and from CCS to GND, respectively.
When regulating the charge voltage, the MAX1870A
behaves as a current-mode step-down or step-up
power supply. Since a current-mode controller regulates
its output current as a function of the error signal, the
duty-cycle modulator can be modeled as a GM stage
(Figure 9). Results are similar in step-down, step-up, or
step-up/down, with the exception of a load-dependent
right-half-plane zero that occurs in step-up mode.
The required compensation network is a pole-zero pair
formed with C
enough that its impedance is relatively small compared
to R
gain of the error amplifier near crossover. R
C
fore, the closed-loop response of the system and the
response time upon battery removal.
R
charger’s output capacitor (C
charger output load, R
The equivalent output impedance of the GMV amplifier,
R
transconductance (GMV = ΔI
inversely with the number of cells. GMV = 0.1µA/mV for
four cells, 0.133µA/mV for three cells, and 0.2µA/mV for
two cells. The DC-DC converter’s transconductance
depends upon the charge current-sense resistor RS2:
where A
Application Circuits, so GM
ESR
OUT
OGMV
CV
is the equivalent series resistance (ESR) of the
determine the crossover frequency and, there-
at frequencies near crossover. R
, is greater than 10MΩ. The voltage loop
CSI
= 18, and RS2 = 30mΩ in the Typical
CV
GM
and R
PWM
______________________________________________________________________________________
=
L
CV
Voltage Loop Compensation
A
= ΔV
. C
CSI
PWM
Switching Frequency
OUT
CV
1
x RS
BATT
CCV
= 1.85A/V.
is chosen to be large
). R
2
Compensation
/ ΔI
L
/ ΔV
is the equivalent
CHG
BATT
CV
IN
= R
) scales
between
sets the
CV
BATT
and
.
Use the following equation to calculate the loop transfer
function (LTF):
The poles and zeros of the voltage-loop transfer func-
tion are listed from lowest frequency to highest frequen-
cy in Table 3.
Near crossover, C
R
inates the parallel impedance near crossover.
Additionally, R
C
C
C
crossover, so the parallel impedance is mostly capaci-
tive and:
If R
a negligible effect near crossover and the loop transfer
function can be simplified as follows:
Figure 9. CCV Simplified Loop Diagram
OGMV
CV
CV
OUT
R
ESR
, so:
OGMV
and dominates the series combination of R
LTF
(
1
also has a much lower impedance than R
(
1
. Since C
Li+ Battery Charger
+
is small enough, its associated output zero has
CCV
R
Step-Up/Step-Down
+
C
CV
=
sC
CV
x
sC
GM
(
R
1
OUT
(
1
CV
L
+
CV
+
PWM
CV
R
sC
x R
x R
sC
O
has a much higher impedance than
CV
R
OUT
is in parallel with R
CV
x
OGMV
L
L
)
has much lower impedance than
GMV
R
x R
x G
GM
x R
OGMV
(
1
)
OUT
CV
MV
L
+
)
)
sC
≅
x
x
≅
REF
(
CV
sC
(
1
R
1
CV
+
+
1
OUT
x R
BATT
sC
,
sC
near crossover
OGMV,
OGMV
OUT
CV
R
R
C
ESR
x R
)
CV
OUT
C
)
ESR
CV
x
CV
L
)
dom-
R
near
L
and
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