Rainbow Electronics MAX5098A - Manual (Page 14)

MAX5098A
Dual, 2.2MHz, Automotive Buck or Boost
Converter with 80V Load-Dump Protection
14 ______________________________________________________________________________________
Detailed Description
PWM Controller
The MAX5098A dual DC-DC converter uses a pulse-
width-modulation (PWM) voltage-mode control scheme.
On each converter the device includes one integrated
n-channel MOSFET switch and requires an external
low-forward-drop Schottky diode for output rectifica-
tion. The controller generates the clock signal by divid-
ing down the internal oscillator (f
CKO ) or the SYNC
input when driven by an external clock, therefore each
controller’s switching frequency equals half the oscilla-
tor frequency (f
SW = f CKO /2) or half of the SYNC input
frequency (f SW = f SYNC /2). An internal transconduc-
tance error amplifier produces an integrated error volt-
age at COMP_, providing high DC accuracy. The
voltage at COMP_ sets the duty cycle using a PWM
comparator and a ramp generator. At each rising edge
of the clock, converter 1’s MOSFET switch turns on and
remains on until either the appropriate or maximum
duty cycle is reached, or the maximum current limit for
the switch is reached. Converter 2 operates 180° out-
of-phase, so its MOSFET switch turns on at each falling
edge of the clock.
In the case of buck operation (see the Typical
Application Circuit ), the internal MOSFET is used in
high-side configuration. During each MOSFET’s on-
time, the associated inductor current ramps up. During
the second half of the switching cycle, the high-side
MOSFET turns off and forward biases the Schottky rec-
tifier. During this time, the SOURCE_ voltage is
clamped to a diode drop (V
D ) below ground. A low for-
ward voltage drop (0.4V) Schottky diode must be used
to ensure the SOURCE_ voltage does not go below
-0.6V abs max. The inductor releases the stored energy
as its current ramps down, and provides current to the
output. The bootstrap capacitor is also recharged when
the SOURCE_ voltage goes low during the high-side
MOSFET off-time. The maximum duty-cycle limit
ensures proper bootstrap charging at startup or low
input voltages. The circuit goes in discontinuous con-
duction mode operation at light load, when the inductor
current completely discharges before the next cycle
commences. Under overload conditions, when the
inductor current exceeds the peak current limit of the
respective switch, the high-side MOSFET turns off
quickly and waits until the next clock cycle.
In the case of boost operation, the MOSFET is a low-
side switch (Figure 6). During each on-time, the induc-
tor current ramps up. During the second half of the
switching cycle, the low-side switch turns off and for-
ward biases the Schottky diode. During this time, the
DRAIN_ voltage is clamped to a diode drop (V
D ) above
V OUT_ and the inductor provides energy to the output
as well as replenishes the output capacitor charge.
Load-Dump Protection
Most automotive applications are powered by a multi-
cell, 12V lead-acid battery with a voltage from 9V to
16V (depending on load current, charging status, tem-
perature, battery age, etc.). The battery voltage is dis-
tributed throughout the automobile and is locally
regulated down to voltages required by the different
system modules. Load dump occurs when the alterna-
tor is charging the battery and the battery becomes
disconnected. Power in the alternator inductance flows
into the distributed power system and elevates the volt-
age seen at each module. The voltage spikes have rise
times typically greater than 5ms and decays within sev-
eral hundred milliseconds but can extend out to 1s or
more depending on the characteristics of the charging
system. These transients are capable of destroying
sensitive electronic equipment on the first fault event.
During load dump, the MAX5098A provides the ability
to clamp the input-voltage rail of the internal DC-DC
converters to a safe level, while preventing power dis-
continuity at the DC-DC converters’ outputs.
The load-dump protection circuit utilizes an internal
charge pump to drive the gate of an external n-channel
MOSFET. This series protection MOSFET absorbs the
load-dump overvoltage transient and operates in satu-
ration over the normal battery range to minimize power
dissipation. During load dump, the gate voltage of the
protection MOSFET is regulated to prevent the source
terminal from exceeding 19V.
The DC-DC converters are powered from the source
terminal of the load-dump protection MOSFET, so that
their input voltage is limited during load-dump and can
operate normally.
ON/OFF
The MAX5098A provides an input (ON/OFF) to turn on
and off the external load-dump protection MOSFET.
Drive ON/OFF high for normal operation. Drive ON/OFF
low to turn off the external n-channel load-dump protec-
tion MOSFET and reduce the supply current to 7µA (typ).
When ON/OFF is driven low, the converter also turns off,
and the PGOOD_ outputs are driven low. V+ will be self
discharged through the converters output currents and
the IC supply current.