MAX17020ETJ+T Maxim Integrated Products, MAX17020ETJ+T Datasheet - Page 22

• Manufacturer Part Number: MAX17020ETJ+T
• Description: IC CTLR PWM DUAL STEP DN 32-TQFN
• Manufacturer: Maxim Integrated Products
• Series: Quick-PWM™r

Specifications of MAX17020ETJ+T

• Applications: Power Supplies
• Current - Supply: 1mA
• Voltage - Supply: 6 V ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The low-noise forced-PWM mode (SKIP = V
ables the zero-crossing comparator, which controls the
low-side switch on-time. This forces the low-side gate-
drive waveform to constantly be the complement of the
high-side gate-drive waveform, so the inductor current
reverses at light loads while DH maintains a duty factor
of V
keep the switching frequency fairly constant. However,
forced-PWM operation comes at a cost: the no-load 5V
bias current remains between 20mA to 60mA depend-
ing on the switching frequency and MOSFET selection.
The MAX17020 automatically uses forced-PWM opera-
tion during all transitions—dynamic REFIN, startup, and
shutdown—regardless of the SKIP configuration.
In skip mode (SKIP = GND), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is affected by a comparator that truncates
the low-side switch on-time at the inductor current’s
zero crossing. The zero-crossing comparator threshold
is set by the differential across LX and AGND.
DC output-accuracy specifications refer to the integrat-
ed threshold of the error comparator. When the inductor
is in continuous conduction, the MAX17020 regulates
the valley of the output ripple and the internal integrator
removes the actual DC output-voltage error caused by
the output-ripple voltage and internal slope compensa-
tion. In discontinuous conduction (SKIP = GND and
I
the low-frequency output ripple error, so the output volt-
age has a DC regulation level higher than the error
comparator threshold by approximately 1.5% due to
slope compensation and output ripple voltage.
Leaving SKIP unconnected or connecting SKIP to REF
(2V) activates a unique pulse-skipping mode with a
guaranteed minimum switching frequency of 20kHz.
This ultrasonic pulse-skipping mode eliminates audio-
frequency modulation that would otherwise be present
when a lightly loaded controller automatically skips
pulses. In ultrasonic mode, the controller automatically
transitions to fixed-frequency PWM operation when the
load reaches the same critical conduction point
(I
An ultrasonic pulse occurs (Figure 4) when the con-
troller detects that no switching has occurred within the
last 37μs or when SECFB drops below its feedback
threshold. Once triggered, the ultrasonic circuitry pulls
Dual Quick-PWM Step-Down Controller
with Low-Power LDO, RTC Regulator
22
OUT
LOAD(SKIP)
OUT
______________________________________________________________________________________
< I
Automatic Pulse-Skipping Mode ( S S K K I I P P = GND)
/V
LOAD(SKIP)
IN
) that occurs when normally pulse skipping.
. The benefit of forced-PWM mode is to
Ultrasonic Mode ( S S K K I I P P = Open or REF)
), the integrator cannot correct for
Forced-PWM Mode ( S S K K I I P P = V
Modes of Operation
CC
) dis-
CC
)
DL high, turning on the low-side MOSFET to induce a
negative inductor current. After the inductor current
reaches the negative ultrasonic current threshold, the
controller turns off the low-side MOFET (DL pulled low)
and triggers a constant on-time (DH driven high). When
the on-time has expired, the controller reenables the
low-side MOSFET until the inductor current drops below
the zero-crossing threshold. Starting with a DL pulse
greatly reduces the peak output voltage when com-
pared to starting with a DH pulse.
The output voltage at the beginning of the ultrasonic
pulse determines the negative ultrasonic current thresh-
old, resulting in the following equation:
where V
age, and V
V
across LX to AGND.
When the controller skips pulses (SKIP = GND or REF),
the long time between pulses (especially if the output is
sinking current) allows the external charge-pump voltage
or transformer secondary winding voltage to drop. When
the SECFB voltage drops below its 2V feedback thresh-
old, the MAX17020 issues an ultrasonic pulse (regardless
of the ultrasonic one-shot state). This forces a switching
cycle, allowing the external unregulated charge pump (or
transformer secondary winding) to be refreshed. See the
Ultrasonic Mode (SKIP = Open or REF) section for
switching cycle sequence/specifications.
Figure 4. Ultrasonic Waveforms
NOM
), and R
V
NEG(US)
NOM
0
Secondary Feedback: SECFB—OUT1 ONLY
I
SONIC
FB
is the nominal feedback-regulation volt-
CS
is the actual feedback voltage (V
= I
37μs (typ)
is the current-sense resistance seen
L
R
CS
= (V
ZERO-CROSSING
DETECTION
NOM
ON-TIME (t
- V
FB
ON
) x 0.385V
)
INDUCTOR
CURRENT
FB
>