MAX1846/MAX1847
High-Efficiency, Current-Mode,
Inverting PWM Controller
8 _______________________________________________________________________________________
Pin Description
PIN
MAX1846 MAX1847
NAME FUNCTION
— 1 POL
Sets polarity of the EXT pin. Connect POL to GND to set EXT for use with an external
PMOS high-side FET. Connect POL to VL to set EXT for use with an external NMOS low-
side FET in transformer-based applications.
1 2 VL VL Low-Dropout Regulator. Connect 0.47µF ceramic capacitor from VL to GND.
2 3 FREQ
Oscillator Frequency Set Input. Connect a resistor (R
FREQ
) from FREQ to GND to set the
internal oscillator frequency from 100kHz (R
FREQ
= 500kΩ) to 500kHz (R
FREQ
= 76.8kΩ).
R
FREQ
is still required if an external clock is used at SYNC. See Setting the Operating
Frequency section.
3 4 COMP
Compensation Node for Error Amp/Integrator. Connect a series resistor/capacitor network
from COMP to GND for loop compensation. See Design Procedure.
4 5 REF
1.25V Reference Output. REF can source up to 500µA. Bypass with a 0.1µF ceramic
capacitor from REF to GND.
56FB
Feedback Input. Connect FB to the center of a resistor-divider connected between the
output and REF. The FB threshold is 0.
— 7, 9 N.C. No Connection
—8SHDN
Shutdown Control. Drive SHDN low to turn off the DC-DC controller. Drive high or connect
to IN for normal operation.
6 10, 11 GND Analog Ground. Connect to PGND.
7 12 PGND Negative Rail for EXT Driver and Negative Current-Sense Input. Connect to GND.
813CS
P osi ti ve C ur r ent- S ense Inp ut. C onnect a cur r ent- sense r esi stor ( R
C S
) b etw een C S and
9 14 EXT External MOSFET Gate-Driver Output. EXT swings from IN to PGND.
10 15 IN Power-Supply Input
— 16 SYNC
Operating Frequency Synchronization Control. Drive SYNC low or connect to GND to set
the internal oscillator frequency with R
FREQ
. Drive SYNC with a logic-level clock input
signal to externally set the converter’s operating frequency. DC-DC conversion cycles
initiate on the rising edge of the input clock signal. Note that when driving SYNC with an
external signal, R
FREQ
must still be connected to FREQ.
