MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
10 ______________________________________________________________________________________
Detailed Description
The MAX1864/MAX1865 power-supply controllers pro-
vide system power for cable and xDSL modems. The
main step-down DC-DC controller operates in a cur-
rent-mode pulse-width-modulation (PWM) control
scheme to ease compensation requirements and pro-
vide excellent load- and line-transient response.
The MAX1864 includes two analog gain blocks to regu-
late two additional positive auxiliary output voltages,
and the MAX1865 includes four analog gain blocks to
regulate three additional positive and one negative aux-
iliary output voltages. The positive regulator gain blocks
can be used to generate low-voltage rails directly from
the main step-down converter or higher voltages using
coupled windings from the step-down converter. The
negative gain block can be used in conjunction with a
coupled winding to generate -5V, -12V, or -15V.
DC-DC Controller
The MAX1864/MAX1865 step-down converters use a
pulse-width-modulated (PWM) current-mode control
scheme (Figure 2). An internal transconductance
amplifier establishes an integrated error voltage at the
COMP pin. The heart of the current-mode PWM con-
troller is an open-loop comparator that compares the
integrated voltage-feedback signal against the ampli-
fied 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, sourc-
ing current to the output and storing energy in a mag-
netic 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 current), the circuit acts as a switch-
mode transconductance amplifier. It pushes the output
LC filter pole, normally found in a voltage-mode PWM,
to a higher frequency. To preserve inner loop stability
and eliminate inductor stair-casing, a slope-compensa-
tion 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 output.
Therefore, the output capacitor stores charge when the
inductor current exceeds the load current and dis-
charges when the inductor current is lower, smoothing
Pin Description (continued)
NAME FUNCTION
9 13 ILIM
Dual Mode Current-Limit Adjustment Input. Connect to VL for the default 250mV
current-limit threshold. In adjustable mode, the current-limit threshold voltage is 1/5th
the voltage present at ILIM. Connect to a resistive-divider between VL and GND to
adjust V
ILIM
between 1V and 2.5V. The logic threshold for switchover to the 250mV
default value is approximately VL - 1V.
10 14 GND Ground
11 15 DL Low-Side Gate-Driver Output. DL swings between GND and VL.
12 16 LX
Inductor Connection. Used for current sense between IN and LX, and used for current
limit between LX and GND.
13 17 DH High-Side Gate-Driver Output. DH swings between LX and BST.
14 18 BST
Boost Flying Capacitor Connection. Connect BST to the external boost diode and
capacitor as shown in the standard application circuit (Figures 1 and 6).
15 19 VL
Internal 5V Linear-Regulator Output. Supplies the IC and powers the DL low-side gate
driver and external boost diode and capacitor. Bypass with a 1µF or greater ceramic
capacitor to GND.
16 20 IN
Input Supply Voltage, 4.5V to 28V. Bypass to GND with a 1µF or greater ceramic
capacitor close to the IC.
