MAX1870AETJ+ Maxim Integrated Products, MAX1870AETJ+ Datasheet - Page 25

• 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

loop (CCS) is in control. Since the output capacitor’s
impedance has little effect on the response of the current
loop, only a single pole is required to compensate this
loop. A
sense amplifiers. RS2 is the charge current-sense resis-
tor. RS1a and RS1b are the adapter current-sense
resistors. R
impedance of the GMI and GMS amplifiers, which are
greater than 10MΩ. GMI is the charge-current amplifier
transconductance (2.4µA/mV). GMS is the adapter-cur-
rent amplifier transconductance (1.7µA/mV.) GM
the DC-DC converter transconductance (1.85A/V).
Use the following equation to calculate the loop transfer
function:
which describes a single-pole system. Since GM
the loop-transfer function simplifies to:
Use the following equations to calculate the crossover
frequency:
Figure 10. CCV Loop Response
LTF
=
CSI
GM
-20
-40
80
60
40
20
0
f
1.E-01
and A
PWM
CO CI
OGMI
_
LTF
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06
x A
CSS
A
=
and R
CS _
=
CS
______________________________________________________________________________________
CCV LOOP RESPONSE
PHASE
2
GM
GMI
_
are the internal gains of the current-
π
C
1
FREQUENCY (Hz)
x RS
x RS
OGMS
CI
_
,
1
MAG
f
_
_
CO CS
+
x GM
are the equivalent output
sR
_
R
OGM
OGM
_
=
1
2
GMS
_
+
π
_
C
sR
x C
CS
R
OGM
OGM
C
0
-45
-90
-135
_
_
_
x C
PWM
PWM
C
_
=
is
For stability, choose a crossover frequency lower than
1/10th of the switching frequency and lower than half of
the RHP zero.
C
This zero is inversely proportional to charge current
and may cause the system to go unstable at high cur-
rents when in step-up mode. A right-half-plane zero is
detrimental to both phase and gain. To also ensure sta-
bility under maximum load in step-up mode, the CCI
crossover frequency must also be lower than f
The right-half-plane zero does not affect CCS.
Choosing a crossover frequency of 30kHz and using
the component values listed in Figure 1 yields C
C
times the minimum value may slow down the current
loop response excessively. Figure 13 shows the Bode
Plot of the input-current frequency response using the
values calculated above.
DHI and DBST are optimized for driving moderately-
sized power MOSFETs. Use low-inductance and low-
resistance traces from driver outputs to MOSFET gates.
DHI typically sources 1.6A and sinks 0.8A to or from
the gate of the p-channel MOSFET. DHI swings from
VHP to VHN. VHN is a negative LDO that regulates with
respect to VHP to provide high-side gate drive.
Connect VHP to DCIN. Bypass VHN with a 1µF capaci-
tor to VHP.
Figure 11. CCI Simplified Loop Diagram
f
CI
RHPZ WorstCase
CS_
= 10 GMI / (2π x f
> 10nF. Values for C
_
Li+ Battery Charger
Step-Up/Step-Down
CCI
C
CI
=
2
R
V
π
OGMI
IN MIN
OSC
x L I
_
GM
PWM
GMI
), C
L
CI
=
CS
2
/ C
= 10 GMS / (2π x f
π
REF
L I
MOSFET Drivers
CS
OUTMAX
CSI
V
greater than ten
RS2
A
IN MIN
CSI
_
V
2
OUTMAX
CI
RHPZ
OSC
and
25
)
.