MAX17035(2009) 查看數據表（PDF） - Maxim Integrated
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MAX17035 Datasheet PDF : 27 Pages
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High-Frequency,
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must be high, MAX must be low, and OVP must be low for
the controller to initiate a new cycle. If the peak inductor
current exceeds IMAX comparator threshold or the output
voltage exceeds the OVP threshold, then the on-time is
terminated. The cycle-by-cycle current limit effectively
protects against overcurrent and short-circuit faults.
If during the off-time the inductor current goes to zero,
the ZCMP comparator output pulls high, turning off the
low-side MOSFET. Both the high- and low-side MOSFETs
are turned off until another cycle is ready to begin.
The MAX17035/MAX17435/MAX17535 enter into the
discontinuous conduction mode (see the Discontinuous
Conduction section).
The on-time is calculated according to the following
equation:
t ON
=
L × IRIPPLE
VCSSN - VBATT
where:
IRIPPLE
=
VBATT ×
L
t OFF
There is a 0.3Fs minimum off-time when the (VDCIN -
VBATT) differential becomes too small. If VBATT R 0.88
x VDCIN, then the threshold for minimum off-time is
reached and the off-time is fixed at 0.27Fs. The switching
frequency in this mode varies according to the equation:
f
=
1
L × IRIPPLE
VCSSN - VBATT
+
t OFF
Discontinuous Conduction
The MAX17035/MAX17435/MAX17535 can also operate
in discontinuous conduction mode to ensure that the
inductor current is always positive. The MAX17035/
MAX17435/MAX17535 enter discontinuous conduction
mode when the output of the LVC control point falls below
150mV. For RS2 = 10mI, this corresponds to 375mA:
IDIS
=
1
2
×
150mV
20 × RS2
=
375mA
where charge current for RS2 = 10mI.
In discontinuous mode, a new cycle is not started until
the LVC voltage rises above 150mV. Discontinuous
mode operation can occur during conditioning charge of
overdischarged battery packs, when the charge current
has been reduced sufficiently by the CCS control loop,
or when the charger is in constant-voltage mode with a
nearly full battery pack.
Compensation
The CCI loop is internally compensated. The CCV and
the CCS share the external compensation capacitor.
The control loop, which is dominant, uses the external
compensation cap and the one that is not used uses an
internal compensation capacitor.
CCV Loop Compensation
The simplified schematic in Figure 6 is sufficient to
describe the operation of the MAX17035/MAX17435/
MAX17535 when the voltage loop (CCV) is in control.
The required compensation network is a pole-zero pair
formed with CCV and RCV, which is an internal 1.7kI. The
pole is necessary to roll off the voltage loop’s response
at low frequency; CCV = 330pF is sufficient for most
applications.
GMOUT
BATT
RESR
RL
CCV
COUT
GMV
RCV
ROGMV
CCV
REF
Figure 6. CCV Loop Diagram
MOSFET Drivers
The DHI and DLO outputs are optimized for driving
moderate-sized power MOSFETs. The MOSFET drive
capability is the same for both the low-side and high-
sides switches. This is consistent with the variable duty
factor that occurs in the notebook computer environment
where the battery voltage changes over a wide range.
There must be a low-resistance, low-inductance path
from the DLO driver to the MOSFET gate to prevent shoot-
through. Otherwise, the sense circuitry in the MAX17035/
MAX17435/MAX17535 interprets the MOSFET gate as
off while there is still charge left on the gate. Use very
short, wide traces measuring 10 squares to 20 squares
or less (1.25mm to 2.5mm wide if the MOSFET is 25mm
from the device). Unlike the DLO output, the DHI output
uses a 50ns (typ) delay time to prevent the low-side
MOSFET from turning on until DHI is fully off. The same
considerations should be used for routing the DHI signal
to the high-side MOSFET.
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