Rainbow Electronics MAX1847 - Manual (Page 11)

MAX1846/MAX1847
High-Efficiency, Current-Mode,
Inverting PWM Controller
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Detailed Description
The MAX1846/MAX1847 current-mode PWM controller
use an inverting topology that is ideal for generating
output voltages from -2V to -200V. Features include
shutdown, adjustable internal operating frequency or
synchronization to an external clock, soft-start,
adjustable current limit, and a wide (+3V to +16.5V)
input range.
PWM Controller
The architecture of the MAX1846/MAX1847 current-
mode PWM controller is a Bi-CMOS multi-input system
that simultaneously processes the output-error signal,
the current-sense signal, and a slope-compensation
ramp (Functional Diagram). Slope compensation pre-
vents subharmonic oscillation, a potential result in cur-
rent-mode regulators operating at greater than 50%
duty cycle. The controller uses fixed-frequency, cur-
rent-mode operation where the duty ratio is set by the
input-to-output voltage ratio. The current-mode feed-
back loop regulates peak inductor current as a function
of the output error signal.
Internal Regulator
The MAX1846/MAX1847 incorporate an internal low-
dropout regulator (LDO). This LDO has a 4.25V output
and powers all MAX1846/MAX1847 functions (exclud-
ing EXT) for the primary purpose of stabilizing the per-
formance of the IC over a wide input voltage range
(+3V to +16.5V). The input to this regulator is connect-
ed to IN, and the dropout voltage is typically 100mV, so
that when V
IN is less than 4.35V, VL is typically V IN
minus 100mV. When the LDO is in dropout, the
MAX1846/MAX1847 still operate with V
IN as low as 3V.
For best performance, it is recommended to connect
VL to IN when the input supply is less than 4.5V.
Undervoltage Lockout
The MAX1846/MAX1847 have an undervoltage lockout
circuit that monitors the voltage at VL. If VL falls below
the UVLO threshold (2.8V typ), the control logic turns the
P-channel FET off (EXT high impedance). The rest of the
IC circuitry is still powered and operating. When VL
increases to 60mV above the UVLO threshold, the IC
resumes operation from a start up condition (soft-start).
Soft-Start
The MAX1846/MAX1847 feature a “digital” soft-start
that is preset and requires no external capacitor. Upon
startup, the FB threshold decrements from the refer-
ence voltage to 0 in 64 steps over 1024 cycles of f
OSC
or f SYNC . See the Typical Operating Characteristics for
a scope picture of the soft-start operation. Soft-start is
implemented: 1) when power is first applied to the IC,
2) when exiting shutdown with power already applied,
and 3) when exiting undervoltage lockout.
Shutdown (MAX1847 only)
The MAX1847 shuts down to reduce the supply current
to 10µA when SHDN is low. In this mode, the internal ref-
erence, error amplifier, comparators, and biasing circuit-
ry turn off. The EXT output becomes high impedance
and the external pullup resistor connected to EXT pulls
V
EXT to V IN , turning off the P-channel MOSFET. When in
shutdown mode, the converter’s output goes to 0.
Frequency Synchronization
(MAX1847 only)
The MAX1847 is capable of synchronizing its switching
frequency with an external clock source. Drive SYNC
with a logic-level clock input signal to synchronize the
MAX1847. A switching cycle starts on the rising edge
of the signal applied to SYNC. Note that the frequency
of the signal applied to SYNC must be higher than the
default frequency set by R
FREQ . This is required so that
the internal clock does not start a switching cycle pre-
maturely. If SYNC is inactive for an entire clock cycle of
the internal oscillator, the internal oscillator takes over
the switching operation. Choose R
FREQ such that f OSC
= 0.9 ✕ f SYNC .
EXT Polarity (MAX1847 only)
The MAX1847 features an option to utilize an N-channel
MOSFET configuration, rather than the typical P-chan-
nel MOSFET configuration (Figure 1). In order to drive
the different polarities of these MOSFETs, the MAX1847
is capable of reversing the phase of EXT by 180
degrees. When driving a P-channel MOSFET, connect
POL to GND. When driving an N-Channel MOSFET,
connect POL to VL. These POL connections ensure the
proper polarity for EXT. For design guidance in regard
to this application, refer to the MAX1856 data sheet.
Design Procedure
Initial Specifications
In order to start the design procedure, a few parameters
must be identified: the minimum input voltage expected
(V
IN(MIN) ), the maximum input voltage expected
(V IN(MAX) ), the desired output voltage (V OUT ), and the
expected maximum load current (I LOAD ).
Calculate the Equivalent Load Resistance
This is a simple calculation used to shorten the verifica-
tion equations:
R LOAD = V OUT / I LOAD