MAX1909ETIT Maxim Integrated Products, Inc., MAX1909ETIT Datasheet - Page 22

• Manufacturer Part Number: MAX1909ETIT
• Description: Multichemistry Battery Chargers With Automatic System Power Selector
• Manufacturer: Maxim Integrated Products, Inc.

Multichemistry Battery Chargers with Automatic
System Power Selector
In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 0.15V. Discontinuous-
mode operation can occur during conditioning charge
of overdischarged battery packs, when the charge cur-
rent has been reduced sufficiently by the CCS control
loop, or when the charger is in constant voltage mode
with a nearly full battery pack.
The charge voltage, charge current, and input current-
limit regulation loops are compensated separately and
independently at the CCV, CCI, and CCS pins.
The simplified schematic in Figure 5 is sufficient to
describe the operation of the MAX1909/MAX8725 when
the voltage loop (CCV) is in control. The required com-
pensation network is a pole-zero pair formed with C
and R
loop’s response at low frequency. The zero is necessary
to compensate the pole formed by the output capacitor
and the load. R
(ESR) of the charger output capacitor (C
equivalent charger output load, where R
∆I
amplifier, R
loop transconductance (GMV = I
on the MODE input, which determines the number of
cells. GMV = 0.125mA/mV for 4 cells and GMV =
0.167mA/mV for 3 cells. The DC-DC converter transcon-
ductance is dependent upon the charge current-sense
resistor RS2:
Table 1. Poles and Zeros of the Voltage-Loop Transfer Function
22
CHG
NO.
1
2
3
4
______________________________________________________________________________________
CV
. The equivalent output impedance of the GMV
Output pole
Output zero
. The pole is necessary to roll off the voltage
CCV pole
CCV zero
NAME
OGMV
ESR
, is greater than 10MΩ. The voltage
is the equivalent series resistance
f
f
Z OUT
P CV
f
f
Z CV
P OUT
_
_
_
_
CCV Loop Compensation
CALCULATION
=
=
CCV
=
2π
=
2π
2π
2π
R
R
Compensation
R
/ V
OGMV
CV
R
ESR
L
1
BATT
OUT
1
×
×
1
L
1
C
C
×
= ∆V
×
). R
OUT
C
CV
) depends
C
OUT
CV
L
BATT
is the
CV
/
Lowest frequency pole created by C
resistance. Since R
exact value for the pole frequency is also not well controlled
(R
Voltage-loop compensation zero. If this zero is at the same
frequency or lower than the output pole f
transfer function approximates a single pole response near the
crossover frequency. Choose C
decade below crossover to ensure adequate phase margin.
Output pole formed with the effective load resistance R
output capacitance C
affect the stability of the system or the crossover frequency.
Output ESR Zero. This zero can keep the loop from crossing unity
gain if f
therefore, choose a capacitor with an ESR zero greater than the
crossover frequency.
OGMV
where A
Operating Circuits (Figures 1 and 2), so GM
3.33A/V.
The loop transfer function is:
Figure 5. CCV Loop Diagram
Z_OUT
> 10M
LTF GM
(
1
+
CSI
is less than the desired crossover frequency;
CCV
R
C
Ω
sC
CV
=
CV
).
= 20, and RS2 = 0.015Ω in the Typical
OUT
R
OGMV
L
OUT
OUT
GM
×
R
R
. R
DESCRIPTION
is very large and not well controlled, the
OGMV
×
L
OUT
L
)
R
G
influences the DC gain but does not
OGMV
GMV
MV
CV
GM
(
=
1
OUT
+
to place this zero at least one
(
A
1
CV
sC
CSI
+
× +
sC
and GMV’s finite output
CV
(
1
P_OUT
1
×
REF
OUT
×
RS
sC
BATT
R
, then the loop
OGMV
2
CV
×
R
×
ESR
R
R
C
ESR
OUT
)
CV
L
)
and the
)
×
OUT
R
L
=