LTC3826-1 Datasheet, PDF (18/32 Pages) Linear Technology – 30μA IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3826-1
APPLICATIONS INFORMATION
EXTV_CC LDO regulates it to 7.5V. Each of these can supply
a peak current of 50mA and must be bypassed to ground
with a minimum of 4.7μF ceramic capacitor. The ceramic
capacitor placed directly adjacent to the INTV_CC and
PGND IC pins is highly recommended. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3826-1 to be
exceeded. The INTV_CC current, which is dominated by the
gate charge current, may be supplied by either the 5.25V
V_IN LDO or the 7.5V EXTV_CC LDO. When the voltage on the
EXTV_CC pin is less than 4.7V, the V_IN LDO is enabled. Power
dissipation for the IC in this case is highest and is equal
to V_IN • INTV_CC. The gate charge current is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be estimated
by using the equation given in Note 2 of the Electrical Char-
acteristics. For example, the LTC3826-1 INTV_CC current is
limited to less than 24mA from a 24V supply when in the
G package and not using the EXTV_CC supply:
T_J = 70°C + (24mA)(24V)(95°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above 4.7V, the
V_IN LDO is turned off and the EXTV_CC LDO is enabled. The
EXTV_CC LDO remains on as long as the voltage applied to
EXTV_CC remains above 4.5V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 7.5V, so while EXTV_CC
is less than 7.5V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 7.5V up to an absolute maximum of 10V,
INTV_CC is regulated to 7.5V.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3826-1’s
switching regulator outputs (4.7V ≤ V_OUT ≤ 10V) during
normal operation and from the V_IN LDO when the output
is out of regulation (e.g., start-up, short-circuit). If more
current is required through the EXTV_CC LDO than is speci-
fied, an external Schottky diode can be added between the
EXTV_CC and INTV_CC pins. Do not apply more than 10V to
the EXTV_CC pin and make sure than EXTV_CC ≤ V_IN.
Significant efficiency and thermal gains can be realized
by powering INTV_CC from the output, since the V_IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 10V regulator outputs, this means connecting
the EXTV_CC pin directly to V_OUT. Tying the EXTV_CC pin
to a 5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (24mA)(5V)(95°C/W) = 81°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV_CC power from
the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC Left Open (or Grounded). This will cause
INTV_CC to be powered from the internal 5.25V regulator
resulting in an efficiency penalty of up to 10% at high
input voltages.
2. EXTV_CC Connected directly to V_OUT. This is the normal
connection for a 5V to 10V regulator and provides the
highest efficiency.
3. EXTV_CC Connected to an External supply. If an external
supply is available in the 5V to 10V range, it may be
used to power EXTV_CC providing it is compatible with
the MOSFET gate drive requirements.
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