MAX887 Maxim, MAX887 Datasheet - Page 8

• Manufacturer Part Number: MAX887
• Description: 100% Duty Cycle / Low-Noise / Step-Down / PWM DC-DC Converter
• Manufacturer: Maxim

The MAX887 is a step-down, pulse-width modulation
(PWM) DC-DC converter that provides an adjustable
output from 1.25V to 10.5V. It accepts inputs from 3.5V
to 11V and delivers up to 600mA. An internal MOSFET
and synchronous rectifier reduce PC board area while
maintaining high efficiency. Cycle-by-cycle current lim-
iting protects the internal MOSFETs and reduces sys-
tem stress during overload conditions. Operation with
up to 100% duty cycle for an output of 3V and higher
minimizes dropout voltage. Fixed-frequency PWM oper-
ation reduces interference in sensitive communications
and data-acquisition applications. A SYNC input allows
synchronization to an external clock. When enabled,
Idle Mode™ extends battery life under light loads by
placing the regulator in low quiescent current (200µA
typ) pulse-frequency modulation (PFM) operation.
Shutdown quiescent current is 2.5µA typ.
The MAX887 uses an oscillator-triggered minimum/
maximum on-time current-mode control scheme. The
minimum on-time is approximately 280ns unless in
dropout. The maximum on-time is approximately
4/f
mode feedback provides cycle-by-cycle current limit-
ing for superior load and line response and protection
of the internal MOSFET and rectifier.
At each falling edge of the internal oscillator, the SYNC
cell sends a PWM ON signal to the control and drive
logic, turning on the internal P-channel MOSFET (main
switch) (Figures 1 and 2). This allows current to ramp
up through the inductor (Figure 2) to the load, and
stores energy in a magnetic field. The switch remains
on until either the current-limit (ILIM) comparator is
tripped, the maximum on-time is reached (not shown),
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
Figure 2. Typical Operating Circuit
8
_______________Detailed Description
47 F
OSC
V
OFF
IN
2.2 F
_______________________________________________________________________________________
= 3.5V to 11V
, allowing operation to 100% duty cycle. Current-
ON
0.33 F
0.047 F
V+
REF
SHDN
SYNC
VL
MAX887
GND
V
OUT
= 1.25V (R1/R2 + 1)
LX
FB
PWM Control Scheme
33 H
165k
100k
R1
R2
C1
100pF
V
OUT
= 3.3V
47 F
or the PWM comparator signals that the output is in
regulation. When the switch turns off, during the sec-
ond half of each cycle, the inductor’s magnetic field
collapses, releasing the stored energy and forcing cur-
rent through the output diode to the output filter capaci-
tor and load. The output filter capacitor stores charge
when the inductor current is high and releases it when
the inductor current is low, smoothing the voltage
across the load.
During normal operation, the MAX887 regulates output
voltage by switching at a constant frequency and then
modulating the power transferred to the load per pulse
using the PWM comparator. A multi-input comparator
sums three weighted differential signals (the output
voltage with respect to the reference, the main switch
current sense, and the slope-compensation ramp) and
changes states when a threshold is reached. It modulates
output power by adjusting the inductor peak current
during the first half of each cycle, based on the output
error voltage. The MAX887’s loop gain is relatively low
to enable the use of a small, low-valued output filter
capacitor. The resulting load regulation is 2.5% typ at
500mA. Slope compensation is added to account for
the inductor current waveform’s down slope during the
second half of each cycle, and to eliminate the inductor
current staircasing characteristic of current-mode con-
trollers at high duty cycles.
For the internal oscillator frequency, the f
mum on-time exceeds one cycle and permits operation
to 100% duty cycle. As the input voltage drops, the
duty cycle increases until the P-channel MOSFET is
held on continuously and 100% duty cycle is reached.
Dropout voltage in 100% duty cycle is the output cur-
rent multiplied by the on-resistance of the internal
switch and inductor around 300mV (I
PWM mode, subharmonic oscillation can occur near
dropout, but subharmonic voltage ripple is small, since
the ripple current is low. When using synchronization to
an external oscillator, 100% duty cycle is available for
SYNC frequencies higher than f
Although an external Schottky diode is used as the pri-
mary output rectifier, an N-channel synchronous rectifi-
er turns on to reduce power loss across the diode and
improve efficiency. During the second half of each
cycle, when the inductor current ramps below the
threshold set by the NEGLIM comparator or when the
end of the oscillator period is reached, the synchronous
rectifier turns off. This keeps excess current from flowing
100% Duty-Cycle Operation
Synchronous Rectification
OSC
/4.
OUT
= 500mA). In
OSC
/4 maxi-