MAX1981A Maxim Integrated Products, MAX1981A Datasheet - Page 23

• Manufacturer Part Number: MAX1981A
• Description: (MAX1907A / MAX1981A) Quick-PWM Master Controllers
• Manufacturer: Maxim Integrated Products

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ciency vs. load curve, while higher values result in high-
er full-load efficiency (assuming that the coil resistance
remains fixed) and less output voltage ripple. Penalties
for using higher inductor values include larger physical
size and degraded load-transient response, especially
at low input voltage levels.
The current-limit circuit employs a unique “valley” cur-
rent-sensing algorithm that uses a current-sense resis-
tor between CSP and CSN as the current-sensing
element (Figure 1). If the current-sense signal is above
the current-limit threshold, the PWM is not allowed to
initiate a new cycle
current is actively limited, the actual peak current is
greater than the current-limit threshold by an amount
equal to the inductor ripple current. Therefore, the
exact current-limit characteristic and maximum load
Figure 4. "Valley" Current-Limit Threshold Point
Positioned CPU Core Power Supplies (IMVP-IV)
0
0
Δi
Δt
Quick-PWM Master Controllers for Voltage-
ON-TIME
=
V
BATT
______________________________________________________________________________________
- V
L
(Figure
OUT
I
LIMIT(VALLEY)
TIME
2). Since only the “valley”
Current-Limit Circuit
TIME
= I
LOAD(MAX)
I
PEAK
( )
I
LOAD
2 - LIR
2η
= I
PEAK
I
I
I
PEAK
LOAD
LIMIT
/2
capability are a function of the current-sense resis-
tance, inductor value, and battery voltage. When com-
bined with the undervoltage protection circuit, this
current-limit method is effective in almost every circum-
stance.
There is also a negative current limit that prevents
excessive reverse inductor currents when V
ing current. The negative current-limit threshold is set to
approximately 120% of the positive current limit, and
therefore tracks the positive current limit when ILIM is
adjusted.
The current-limit threshold is adjusted with an external
resistor-divider at ILIM. The current-limit threshold volt-
age adjustment range is from 10mV to 150mV. In the
adjustable mode, the current-limit threshold voltage is
precisely 1/10th the voltage seen at ILIM. The threshold
defaults to 50mV when ILIM is connected to V
logic threshold for switchover to the 50mV default value
is approximately V
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the cur-
rent-sense signals seen by LX and GND. Place the IC
close to the low-side MOSFET with short, direct traces,
making a Kelvin sense connection to the source and
drain terminals.
The DH and DL drivers are optimized for driving moder-
ately sized high-side and larger low-side power
MOSFETs. This is consistent with the low duty factor
seen in the notebook CPU environment, where a large
V
circuit monitors the DL output and prevents the high-
side FET from turning on until DL is fully off. There must
be a low-resistance, low-inductance path from the DL
driver to the MOSFET gate in order for the adaptive
dead-time circuit to work properly. Otherwise, the sense
circuitry in the MAX1907A/MAX1981A will interpret the
MOSFET gate as “off” while there is actually charge still
left on the gate. Use very short, wide traces (50 to 100
mils wide if the MOSFET is 1in from the device). The
dead time at the other edge (DH turning off) is deter-
mined by a fixed 35ns internal delay.
The internal pulldown transistor that drives DL low is
robust, with a 0.5Ω (typ) on-resistance. This helps pre-
vent DL from being pulled up due to capacitive cou-
pling from the drain to the gate of the low-side MOSFET
when LX switches from ground to V
high input voltages and long inductive traces may
require additional gate-to-source capacitance to ensure
fast-rising LX edges do not pullup the DL gate driver,
causing shoot-through currents. The capacitive cou-
IN
- V
OUT
differential exists. An adaptive dead-time
MOSFET Gate Drivers (DH, DL)
CC
- 1V.
IN
. Applications with
OUT
CC
is sink-
. The
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