MAX1960/MAX1961/MAX1962
Operating Frequency and Synchronization
The MAX1960/MAX1961/MAX1962 operating frequency
is set externally to either 500kHz or 1MHz. For 500kHz
operation, connect FSET/SYNC to GND, or for 1MHz
operation, connect FSET/SYNC to V
DD
. Alternately, an
external clock from 450kHz to 1.2MHz can be applied
to SYNC.
A clock output (CLKOUT) that is 180° out-of-phase with
the internal clock is also provided. This allows a second
converter to be synchronized, and operate 180° out-of-
phase with the first. To do this, simply connect CLKOUT
of the first converter to FSET/SYNC of the second con-
verter. The first converter can be set internally to 500kHz
or 1MHz for this mode of operation. When the first con-
verter is synchronized to an external clock, CLKOUT is
the inverse of external clock. See the SYNC Timing
Waveform in the
Typical Operating Characteristics
.
Lossless Current Limit
(MAX1960/MAX1961)
To prevent damage in the case of excessive load cur-
rent or a short circuit, the MAX1960/MAX1961 use the
low-side MOSFET’s on-resistance (R
DS(ON)
) for current
sensing. The current is monitored during the on-time of
the low-side MOSFET. If the current-sense voltage
(V
PGND
- V
LX
) rises above the current-limit threshold for
more than 128 clock cycles, the controller turns off. The
controller remains off until the input voltage is removed
or the device is re-enabled with CTL1 and CTL2 (see
the
Setting the Current Limit
section).
Current-Sense Resistor (MAX1962)
The MAX1962 uses a standard current-sense resistor in
series with the inductor for a 10% accurate current-limit
measurement. The current-sense threshold is 50mV. This
provides accurate current sensing at all duty cycles with-
out relying on MOSFET on-resistance. CS connects to
the high-side (inductor side) of the current-sense resistor
and OUT connects to the low-side (output side) of the
current-sense resistor.
The current-sense resistor for the MAX1962 may also be
replaced with a series RC network across the inductor.
This method uses the parasitic resistance of the inductor
for current sensing. This method is less accurate than
using a current-sense resistor, but is lower cost and pro-
vides slightly higher efficiency. See the
Design
Procedure
section for instructions on using this method.
Dropout Performance
The MAX1960/MAX1961/MAX1962 enter dropout when
the input voltage is not sufficiently high to maintain output
regulation. As input voltage is lowered, the duty cycle
increases until it reaches its maximum value, where the
part enters dropout. With a switching frequency of
1MHz, the maximum duty cycle is about 83%. At
500kHz, the duty cycle can increase to about 92%,
resulting in a lower dropout voltage. The duty cycle is
dependent on the input voltage (V
IN
), the output volt-
age (V
OUT
), and the parasitic voltage drops in the
MOSFETs and the inductor (V
DROP(N1)
, V
DROP(N2)
,
V
DROP(L)
). Note that V
DROP(L)
includes the voltage
drop due to the inductor’s resistance, the drop across
the current-sense resistor (if used), and any other resis-
tive voltage drop from the LX switching node to the
point where the output voltage is sensed. The duty
cycle is found from:
Adaptive Dead Time
The MAX1960/MAX1961/MAX1962 DL and DH MOSFET
drivers have an adaptive dead-time circuit to prevent
shoot-through current caused by high- and low-side
MOSFET overlap. This allows a wide variety of MOSFETs
to be used without matching FET dynamic characteris-
tics. The DL driver will not go high until DH drives the
high-side MOSFET gate to within 1V of its source (LX).
The DH output will not go high until DL drives the low-side
MOSFET gate to within 1V of ground.
Design Procedure
Component selection is primarily dictated by the following
criteria:
Input voltage range. The maximum value
(V
IN(MAX)
) must accommodate the worst-case high
input voltage. The minimum value (V
IN(MIN)
) must
account for the lowest input voltage after drops due
to connectors, fuses, and selector switches are con-
sidered.
Maximum load current. There are two values to con-
sider: The
peak load current
(I
LOAD(MAX)
) determines
the instantaneous component stresses and filtering
requirements and is key in determining output capac-
itor requirements. I
LOAD(MAX)
also determines the
inductor saturation rating and the design of the cur-
rent-limit circuit. The
continuous load current
(I
LOAD
)
determines the thermal stresses and is key in deter-
mining input capacitor requirements, MOSFET
requirements, as well as those of other critical heat-
contributing components.
