MAX16930 Datasheet, PDF (15/29 Pages) Maxim Integrated Products – 2MHz, 36V, Dual Buck with Preboost and 20μA Quiescent Current

MAX16930/MAX16931
2MHz, 36V, Dual Buck with Preboost and
20µA Quiescent Current
High-Side Gate-Driver Supply (BST_)
The high-side MOSFET is turned on by closing an inter-
nal switch between BST_ and DH_ and transferring the
bootstrap capacitor’s (at BST_) charge to the gate of
the high-side MOSFET. This charge refreshes when the
high-side MOSFET turns off and the LX_ voltage drops
down to ground potential, taking the negative terminal
of the capacitor to the same potential. At this time the
bootstrap diode recharges the positive terminal of the
bootstrap capacitor.
The selected n-channel high-side MOSFET determines the
appropriate boost capacitance values (C_{BST_} in the Typical
Operating Circuit) according to the following equation:
C _{BST _}
=
Q_G
∆V_{BST _}
where Q_G is the total gate charge of the high-side
MOSFET and DV_{BST_} is the voltage variation allowed on
the high-side MOSFET driver after turn-on. Choose
DV_{BST_} such that the available gate-drive voltage is not
significantly degraded (e.g., DV_{BST_} = 100mV to 300mV)
when determining C_{BST_}.
The boost capacitor should be a low-ESR ceramic
capacitor. A minimum value of 100nF works in most cases.
Current Limiting and Current-Sense
Inputs (OUT_ and CS_)
The current-limit circuit uses differential current-sense
inputs (OUT_ and CS_) to limit the peak inductor current.
If the magnitude of the current-sense signal exceeds the
current-limit threshold (V_LIMIT1,2 = 80mV (typ)), the PWM
controller turns off the high-side MOSFET. The actual
maximum load current is less than the peak current-
limit threshold by an amount equal to half of the inductor
ripple current. Therefore, the maximum load capability is
a function of the current-sense resistance, inductor value,
switching frequency, and duty cycle (V_{OUT_}/V_IN).
For the most accurate current sensing, use a current-
sense shunt resistor (R_SH) between the inductor and the
output capacitor. Connect CS_ to the inductor side of R_SH
and OUT_ to the capacitor side. Dimension R_SH such that
the maximum inductor current (I_L,MAX = I_LOAD,MAX+1/2
I_RIPPLE,PP) induces a voltage of V_LIMIT1,2 across R_SH
including all tolerances.
For higher efficiency, the current can also be measured
directly across the inductor. This method could cause
up to 30% error over the entire temperature range and
requires a filter network in the current-sense circuit. See
the Current-Sense Measurement section.
Voltage Monitoring (PGOOD_)
The MAX16930/MAX16931 include several power moni-
toring signals to facilitate power-supply sequencing and
supervision. PGOOD_ can be used to enable circuits that
are supplied by the corresponding voltage rail, or to turn
on subsequent supplies. Each PGOOD_ goes high (high
impedance) when the corresponding regulator output
voltage is in regulation.
Each PGOOD_ goes low when the corresponding regula-
tor output voltage drops below 15% (typ) or rises above
15% (typ) of its nominal regulated voltage. Connect a
10kI (typ) pullup resistor from PGOOD_ to the relevant
logic rail to level-shift the signal.
PGOOD_ asserts low during soft-start, soft-discharge,
and when either buck converter is disabled (either EN1
or EN2 is low).
Supply Monitoring (INS)
The supply voltage in automotive systems can vary sig-
nificantly and indicate potentially dangerous situations
for the application. Undervoltage transients can indicate
impending loss of power (for example during engine-start
with a weak battery), while overvoltage conditions can
quickly exceed the thermal budget of the application.
The devices include a dedicated battery voltage sensor
at INS to quickly detect overvoltage and undervoltage for
the boost converter.
Connect INS to the center tap of a resistive divider from
the input voltage (battery) to TERM to set the threshold
voltage for V_INS,OFF, V_INS,ON,SW, and V_INS,UV. For
example, with a 153kI/±1% resistor between INS and
V_BAT and a 20kI/±1% resistor between INS and TERM,
the following typical automotive V_BAT levels can be
sensed, allowing for proper turn-on/turn-off of the pre-
boost. If this setting is not sufficient, optimize the divider
for the most critical level.
SIGNAL
VINS,OFF
VINS,ON,SW
VINS,UV
Rising
VINS,UV
Falling
VBAT(MIN)
(V)
10.38
9.515
2.81
2.38
VBAT(TYP)
(V)
10.81
9.95
3.0275
2.6
VBAT(MAX)
(V)
11.25
10.38
3.24
2.81
Maxim Integrated
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