MAX1992_05 Datasheet, PDF (12/36 Pages) Maxim Integrated Products – Quick-PWM Step-Down Controllers

Quick-PWM Step-Down Controllers with Inductor
Saturation Protection and Dynamic Output Voltages
PIN
MAX1992 MAX1993
3
3
4
4
5
5
6
6
—
7
—
8
9
9
10
10
11
11
Pin Description (continued)
NAME
LSAT
PGOOD
ILIM
REF
REFIN
OD
FB
OUT
CSP
FUNCTION
Inductor Saturation Control Input. This four-level logic input sets the inductor current
saturation limit as a multiple of the valley current-limit threshold set by ILIM, or
disables the function if not required. Connect LSAT to the following pins to set the
saturation current limit:
V_CC = 2 × I_LIM(VAL)
Open = 1.75 × I_LIM(VAL)
REF = 1.5 × I_LIM(VAL)
AGND = disable LSAT protection
See the Inductor Saturation Limit and Setting the Current Limit sections.
Open-Drain Power-Good Output. PGOOD is low when the output voltage is more than
10% (typ) above or below the normal regulation point, during soft-start, and in
shutdown. After the soft-start circuit has terminated, PGOOD becomes high
impedance if the output is in regulation. For the MAX1993, PGOOD is
blanked—forced high-impedance state—when FBLANK is enabled and the controller
detects a transition on GATE.
Valley Current-Limit Threshold Adjustment. The valley current-limit threshold defaults
to 50mV if ILIM is tied to V_CC. In adjustable mode, the valley current-limit threshold
across CSP and CSN is precisely 1/10th the voltage seen at ILIM over a 250mV to
2.5V range. The logic threshold for switchover to the 50mV default value is
approximately V_CC - 1V. When the inductor saturation protection threshold is
exceeded, ILIM sinks 6µA. See the Current-Limit Protection (ILIM) section.
2.0V Reference Voltage Output. Bypass REF to analog ground with a 0.1µF or greater
ceramic capacitor. The reference can source up to 50µA for external loads. Loading
REF degrades output voltage accuracy according to the REF load regulation error.
The reference is disabled when the MAX1992/MAX1993 is shut down.
External Reference Input. REFIN sets the feedback regulation voltage (V_FB = V_REFIN)
of the MAX1993.
Open-Drain Output. Controlled by GATE.
Feedback Input.
MAX1992: Connect to V_CC for a +1.8V fixed output or to AGND for a +2.5V fixed
output. For an adjustable output (0.7V to 5.5V), connect FB to a resistive divider from
the output voltage. The FB regulation level is +0.7V.
MAX1993: The FB regulation level is set by the voltage at REFIN.
Output Voltage Sense. Connect directly to the positive terminal of the output
capacitors as shown in the standard application circuits (Figures 1 and 9). OUT
senses the output voltage to determine the on-time for the high-side switching
MOSFET. For the MAX1992, OUT also serves as the feedback input when using the
preset internal output voltages as shown in Figure 7. When discharge mode is
enabled by OVP/UVP, the output capacitor is discharged through an internal 10Ω
resistor connected between OUT and ground.
Positive Current-Sense Input. Connect to the positive terminal of the current-sense
element. Figure 10 and Table 7 describe several current-sensing options. The PWM
controller does not begin a cycle unless the current sensed is less than the valley
current-limit threshold programmed at ILIM.
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