Datasheet

The ILIM pin is a dual-mode input. When ILIM is con-
nected to VL, a default low-side current limit of 250mV
typ) is used. If ILIM is connected to a voltage between
250mV and 3V, the low-side current limit is typically 1/5th
the ILIM voltage.
The MAX1530/MAX1531s’ current limits are compara-
tively inaccurate, since the maximum load current is a
function of the MOSFETs’ on-resistances and the induc-
tor value, as well as the accuracy of the two thresholds.
However, using MOSFET current sensing reduces both
cost and circuit size and increases efficiency, since sense
resistors are not needed.
MOSFET Gate Drivers (DH, DL)
The DH and DL drivers are optimized for driving mod-
erate-size high-side and low-side MOSFETs. Adaptive
dead-time circuits monitor the DL and DH drivers and
prevent either FET from turning on until the other is fully
off. This algorithm allows operation without shoot- through
with a wide range of MOSFETs, minimizing delays and
maintaining efficiency. When the gates are turning off,
there must be low-resistance, low-induc- tance paths
from the gate drivers to the MOSFET gates for the
adaptive dead-time circuit to work properly. Otherwise,
the sense circuitry in the MAX1530/ MAX1531 interpret
the MOSFET gate as “off” while gate charge actually
remains. Use short, wide traces mea- suring less than 50
squares (at least 20 mil wide if the MOSFET is 1in from
the device). It is advantageous to slow down the turn-on
of both gate drivers if there is noise coupling between the
switching regulator and the linear regulators. The noise
coupling can result in excessive switching ripple on the
linear regulator outputs. Slowing down the turn-on of the
gate drivers proves to be an effective way of reduc- ing
the output ripple. Take care to ensure that the turn- off
times are not affected at the same time. As explained
above, slowing down the turn-off times may result in
shoot-through problems. In Figure 1, a 10Ω resistor (R5)
is inserted in series with the BST pin to slow down the
turn-on of the high-side MOSFET (N1-B) without affecting
the turn-off. A 10Ω resistor (R6) is also inserted between
DL and the gate of the low-side MOS- FET (N1-A) to slow
its turn-on. Because the gate resis- tor would slow down
the turn-off time, connect a switching diode (D2) (such
as 1N4148) in parallel with the gate resistor as shown in
Figure 1 to prevent poten- tial shoot-through.
High-Side Gate-Drive Supply (BST)
A flying-capacitor bootstrap circuit generates gate- drive
voltage for the high-side N-channel switch (Figure 1). The
capacitor C5 between BST and LX is alternately charged
from the VL supply and placed parallel to the high-side
MOSFET’s gate-source terminals.
On startup, the synchronous rectifier (low-side MOS-
FET) forces LX to ground and charges the boost capaci-
tor from VL through diode D1. On the second half-cycle,
the switch-mode power supply turns on the high-side
MOSFET by closing an internal switch between BST and
DH. This provides the necessary gate-to-source voltage
to turn on the high-side switch, an action that boosts the
5V gate-drive signal above the input voltage.
Oscillator Frequency Selection (FREQ)
The FREQ pin can be used to select the switching fre-
quency of the step-down regulator. Connect FREQ to
VL for 500kHz operation. Connect FREQ to AGND for
250kHz operation. The 500kHz operation minimizes the
size of the inductor and capacitors. The 250kHz opera-
tion improves efficiency by 2% to 3%.
Linear Regulator Controllers
The MAX1530/MAX1531 include three positive linear
regulator controllers, LR1, LR2, and LR3. These linear
regulator controllers can be used with external pass tran-
sistors to regulate supplies for TFT LCDs. The MAX1531
includes an additional positive linear regula- tor controller
(LR4) and a negative linear regulator con- troller (LR5).
Low-Voltage Logic Regulator Controller (LR1)
LR1 is an analog gain block with an open-drain N- chan-
nel output. It drives an external PNP pass transis- tor
with a 6.8kΩ base-to-emitter resistor. Its guaranteed base
drive sink current is at least 3mA. The regulator including
transistor Q1 in Figure 1 uses a 10µF output capacitor
and is designed to deliver 500mA at 2.5V.
LR1 is typically used to generate low-voltage logic sup-
plies for the timing controller and the digital sections of the
TFT LCD source/gate driver ICs.
LR1 is enabled when the soft-start of the main step-
down regulator is complete. (See the Startup Sequence
(ONL_,SEQ) section.) Each time it is enabled, the con-
troller goes through a soft-start routine that ramps up its
internal reference DAC. (See the Soft-Start section.)
Gamma Regulator Controller (LR2) LR2
is an analog gain block with an open-drain N- channel
output. It drives an external PNP pass transis- tor with a
6.8kΩ base-to-emitter resistor. Its guaranteed base drive
sink current is at least 2mA. The regulator including tran-
sistor Q2 in Figure 1 uses a 0.47µF output capacitor and
is designed to deliver 50mA at 9.7V.
MAX1530/MAX1531 Multiple-Output Power-Supply
Controllers for LCD Monitors
www.maximintegrated.com
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