MAX1540A Datasheet, PDF (23/49 Pages) Maxim Integrated Products – Dual Step-Down Controllers with Saturation Protection, Dynamic Output, and Linear Regulator

Dual Step-Down Controllers with Saturation
Protection, Dynamic Output, and Linear Regulator
nected internally to the fixed 5V linear-regulator output
(LDOOUT).
The 5V bias supply must provide V_CC (PWM controller)
and V_DD (gate-drive power), so the maximum current
drawn is:
I_BIAS = I_CC + f_SW (Q_G(LOW) + Q_G(HIGH))
= 4mA to 50mA (typ)
where I_CC is 1.1mA (typ), f_SW is the switching frequency,
and Q_G(LOW) and Q_G(HIGH) are the MOSFET data
sheet’s total gate-charge specification limits at V_GS = 5V.
The V+ battery input and 5V bias inputs (V_CC and V_DD)
can be connected together if the input source is a fixed
4.5V to 5.5V supply. If the 5V bias supply powers up
prior to the battery supply, the enable signals (ON1 and
ON2 going from low to high) must be delayed until the
battery voltage is present in order to ensure startup.
Free-Running, Constant On-Time, PWM
Controller with Input Feed Forward
The Quick-PWM control architecture is a pseudofixed-
frequency, constant on-time, current-mode regulator
with voltage feed forward (Figure 2). This architecture
relies on the output filter capacitor’s ESR to act as a
current-sense resistor, so the output ripple voltage pro-
vides the PWM ramp signal. The Quick-PWM algorithm
is simple: the high-side switch on-time relies solely on
an adjustable one-shot whose pulse width is inversely
proportional to input voltage and directly proportional to
output voltage. Another one-shot sets a fixed minimum
off-time (400ns typ). The controller triggers the on-time
one-shot when the error comparator is low, the inductor
current is below the valley current-limit threshold, and
the minimum off-time one-shot has timed out.
On-Time One-Shot (TON)
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the on-time in
response to the battery and output voltages. The high-
side switch on-time is inversely proportional to the bat-
tery voltage as measured by the V+ input (V_IN = V+),
and proportional to the output voltage as measured by
the OUT_ input:
On- Time
=
K
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
where K (switching period) is set by the TON pin-strap
connection (Table 3). This algorithm results in a nearly
constant switching frequency despite the lack of a fixed-
frequency clock generator. The benefits of a constant
switching frequency are twofold: 1) the frequency can
be selected to avoid noise-sensitive regions such as the
455kHz IF band and 2) the inductor ripple-current oper-
ating point remains relatively constant, resulting in easy
design methodology and predictable output voltage rip-
ple. The on-time for the main controller (DH1) is set 15%
higher than the nominal frequency setting (200kHz,
300kHz, 420kHz, or 540kHz), while the on-time for the
secondary controller (DH2) is set 15% lower than the
nominal setting. This prevents audio-frequency “beat-
ing” between the two asynchronous regulators.
The on-time one-shot has good accuracy at the operat-
ing points specified in the Electrical Characteristics
(approximately ±12.5% at 540kHz and 420kHz nominal
settings, and ±10% with the 300kHz and 200kHz set-
tings). On-times at operating points far removed from
the conditions specified in the Electrical Characteristics
can vary over a wider range.
The constant on-time translates only roughly to a constant
switching frequency. The on-times guaranteed in the
Electrical Characteristics are influenced by resistive loss-
es and by switching delays in the high-side MOSFET.
Resistive losses—including the inductor, both MOSFETs,
and PC board copper losses in the output and ground—
tend to raise the switching frequency as the load increas-
es. The dead-time effect increases the effective on-time,
reducing the switching frequency as one or both dead
times add to the effective on-time. It occurs only in PWM
mode (SKIP = V_CC) and during dynamic output-voltage
transitions when the inductor current reverses at light- or
negative-load currents. With reversed inductor current,
the inductor’s EMF causes LX_ to go high earlier than
normal, extending the on-time by a period equal to the
driver dead time.
For loads above the critical conduction point, where the
dead-time effect no longer occurs, the actual switching
frequency is:
( ) ^fSW
=
V_{OUT_} + V_DROP1
t_ON V_IN + V_DROP1 - V_DROP2
where V_DROP1 is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; V_DROP2 is
the sum of the resistances in the charging path, includ-
______________________________________________________________________________________ 23