MAX1535 Maxim Integrated Products, MAX1535 Datasheet - Page 31

• Manufacturer Part Number: MAX1535
• Description: Highly Integrated Level 2 Smbus Battery Charger
• Manufacturer: Maxim Integrated Products

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This describes a single-pole system. Since:
the loop-transfer function simplifies to:
The crossover frequency is given by:
For stability, choose a crossover frequency lower than
1/10th of the switching frequency:
Choosing a crossover frequency of 30kHz and using the
component values listed in Figure 1 yields C
Values for C
may slow down the current-loop response excessively.
Figure 13 shows the Bode plot of the current-loop fre-
quency response using the values calculated above.
The simplified schematic in Figure 14 is sufficient to
describe the operation of the MAX1535 when the input
current-limit loop (CCS) is in control. Since the output
capacitor’s impedance has little effect on the response
of the input current-limit loop, only a single pole is
required to compensate this loop. A
gain of the current-sense amplifier. R1 is the input cur-
rent-sense resistor, R1 = 10mΩ in the typical application
circuits. R
the GMS amplifier, which is greater than 10MΩ. GMS is
the charge-current amplifier transconductance =
1µA/mV. GM
transconductance = (1/D) × GM
The loop-transfer function is given by:
simplifies to
Since GM
LTF GM
=
OGMS
IN
IN
LTF GMS
CI
IN
=
:
LTF GMI
×
A
greater than 10 times the minimum value
is the DC-to-DC converter’s input-referred
A
CSS
=
C
is the equivalent output impedance of
GM
CSS
CI
=
______________________________________________________________________________________
1
f
CO CI
×
OUT
= GMI / (2π f
×
RS
RSI GMS
_
1
1
1
+
,
=
+
×
SR
=
the loop transfer function
SR
A
2π
CCS Loop Compensation
R
CSI
OGMS
R
GMI
OGMI
OGMS
C
OGMI
OUT
1
CI
1
×
CO-CI
+
RS
−
SR
×
= (1/D) × 3.3A/V.
×
Highly Integrated Level 2 SMBus
CSS
2
C
C
R
OGMS
)
CS
OGMS
CI
is the internal
×
CI
C
CS
> 5.4nF.
The crossover frequency is given by:
For stability, choose a crossover frequency lower than
1/10th of the switching frequency:
Choosing a crossover frequency of 30kHz and using
the component values listed in Figure 1 yields C
5.4nF. Values for CCS greater than 10 times the mini-
mum value may slow down the current-loop response
excessively. Figure 15 shows the Bode plot of the input
current-limit-loop frequency response using the values
calculated above.
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-
side switches. This is consistent with the variable duty
factor that occurs in the notebook computer environ-
ment where the battery voltage changes over a wide
range. An adaptive dead-time circuit monitors the DLO
output and prevents the high-side FET from turning on
until DLO is fully off. There must be a low-resistance,
low-inductance path from the DLO driver to the MOS-
FET gate for the adaptive dead-time circuit to work
properly. Otherwise, the sense circuitry in the MAX1535
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 layout
considerations should be used for routing the DHI sig-
nal to the high-side MOSFET.
Since the transition time for a P-channel switch can be
much longer than an N-channel switch, the dead time
prior to the high-side P-channel MOSFET turning on is
more pronounced than in other synchronous step-down
regulators, which use high-side N-channel switches.
On the high-to-low transition, the voltage on the induc-
tor’s “switched” terminal flies below ground until the
low-side switch turns on. A similar dead-time spike
occurs on the opposite low-to-high transition.
Depending upon the magnitude of the load current,
these spikes usually have a minor impact on efficiency.
The high-side driver (DHI) swings from SRC to 5V
below SRC and has a typical impedance of 1Ω sourc-
ing and 4Ω sinking. The low-side driver (DLO) swings
C
Battery Charger
CS
f
CO CS
= 5×GMS / (2π f
_
=
2π
GMS
C
CS
MOSFET Drivers
OSC
)
CS
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