MAX1965TEEP Maxim Integrated, MAX1965TEEP Datasheet - Page 12

• Manufacturer Part Number: MAX1965TEEP
• Description: Current & Power Monitors & Regulators
• Manufacturer: Maxim Integrated

negative gain block can be used in conjunction with a
coupled winding to generate -5V, -12V, or -15V.
The MAX1964/MAX1965 step-down converters use a
pulse-width-modulated (PWM) current-mode control
scheme (Figure 2). An internal transconductance ampli-
fier establishes an integrated error voltage at the COMP
pin. The heart of the current-mode PWM controller is an
open-loop comparator that compares the integrated
voltage-feedback signal against the amplified current-
sense signal plus the slope compensation ramp. At
each rising edge of the internal clock, the high-side
MOSFET turns on until the PWM comparator trips or the
maximum duty cycle is reached. During this on-time,
current ramps up through the inductor, sourcing current
to the output and storing energy in a magnetic field.
The current-mode feedback system regulates the peak
inductor current as a function of the output voltage error
signal. Since the average inductor current is nearly the
same as the peak inductor current (assuming that the
inductor value is relatively high to minimize ripple cur-
rent), the circuit acts as a switch-mode transconduc-
tance amplifier. It pushes the output LC filter pole,
normally found in a voltage-mode PWM, to a higher fre-
quency. To preserve inner-loop stability and eliminate
inductor stair casing, a slope-compensation ramp is
summed into the main PWM comparator.
During the second-half of the cycle, the high-side MOS-
FET turns off and the low-side N-Channel MOSFET
turns on. Now the inductor releases the stored energy
as its current ramps down, providing current to the out-
put. Therefore, the output capacitor stores charge when
the inductor current exceeds the load current, and dis-
charges when the inductor current is lower, smoothing
the voltage across the load. Under overload conditions
when the inductor current exceeds the selected cur-
rent-limit (see the Current Limit section), the high-side
MOSFET is not turned on at the rising edge of the clock
and the low-side MOSFET remains on to let the inductor
current ramp down.
The MAX1964/MAX1965 operate in a forced-PWM
mode, so even under light loads, the controller main-
tains a constant switching frequency to minimize cross-
regulation errors in applications that use a transformer.
So the low-side gate-drive waveform is the complement
of the high-side gate-drive waveform, which causes the
inductor current to reverse under light loads.
The one MAX1964/MAX1965’s one current-sense circuit
amplifies (A
generated by the high-side MOSFET’s on resistance
Tracking/Sequencing Triple/Quintuple
Power-Supply Controllers
12
______________________________________________________________________________________
V
= 4.9) the current-sense voltage
Current-Sense Amplifier
DC-DC Controller
(R
signal and the internal slope compensation signal are
summed together (V
parator’s inverting input. The PWM comparator turns off
the high-side MOSFET when the V
integrated feedback voltage (V
side MOSFET no further than 5mm from the controller
and connect IN and LX to the MOSFET using Kelvin
sense connections to guarantee current-sense accura-
cy and improve stability.
The current-limit circuit employs a unique “valley” cur-
rent-limiting algorithm that uses the low-side MOSFET’s
on-resistance as a sensing element (Figure 3). If the
voltage across the low-side MOSFET (R
I
beginning of a new oscillator cycle, the MAX1964/
MAX1965 will not turn on the high-side MOSFET. The
actual peak current is greater than the current-limit
threshold by an amount equal to the inductor ripple cur-
rent. Therefore, the exact current-limit characteristic
and maximum load capability are a function of the low-
side MOSFET on-resistance, inductor value, input volt-
age, and output voltage. The reward for this uncertainty
is robust, lossless overcurrent limiting.
In adjustable mode, the current-limit threshold voltage is
approximately one-fifth the voltage seen at ILIM (I
= 0.2
necting a resistive-divider from VL to ILIM to GND. The
current-limit threshold can be set from 106mV to
530mV, which corresponds to ILIM input voltages of
500mV to 2.5V. This adjustable current limit accommo-
dates MOSFETs with a wide range of on-resistance
characteristics (see the Design Procedure section). The
current-limit threshold defaults to 250mV when ILIM is
connected to VL. The logic threshold for switchover to
the 250mV default value is approximately VL - 1V.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors don’t corrupt the cur-
rent-sense signals seen by LX and GND. The IC must
be mounted close to the low-side MOSFET with short
(less than 5mm), direct traces making a Kelvin sense
connection.
Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal Schottky catch
diode with a low-resistance MOSFET switch. The
MAX1964/MAX1965 also use the synchronous rectifier
to ensure proper startup of the boost gate-driver circuit
and to provide the current-limit signal.
INDUCTOR
DS(ON)
✕
V
ILIM
✕
) exceeds the current-limit threshold at the
I
). Adjust the current-limit threshold by con-
INDUCTOR
Synchronous Rectifier Driver (DL)
SUM
). This amplified current-sense
) and fed into the PWM com-
COMP
Current-Limit Circuit
SUM
). Place the high-
exceeds the
DS(ON)
VALLEY
✕