MAX1515 Datasheet, PDF (14/24 Pages) Maxim Integrated Products – Low-Voltage, Internal Switch, Step-Down/DDR Regulator

Low-Voltage, Internal Switch,
Step-Down/DDR Regulator
+3.3V Bias Supply (V_CC and V_DD)
The MAX1515 requires a 3.3V bias supply for its inter-
nal circuitry. Typically, this 3.3V bias supply is the note-
book’s 95%-efficient, 3.3V system supply. The 3.3V
bias supply must provide V_CC (PWM controller) and
V_DD (gate-drive and reference buffer power), so the
maximum current drawn is:
I_BIAS = I_CC + I_REFOUT + f_SW (Q_G(LOW) + Q_G(HIGH))
where I_CC is 450µA (typ), f_SW is the switching frequen-
cy, and Q_G(LOW) and Q_G(HIGH) are the internal MOS-
FET total gate charge of approximately 1nC.
The input supply (V_IN) and 3.3V bias inputs (V_CC and
V_DD) can be connected together if the input source is a
fixed 3.0V to 3.6V supply. If the 3.3V bias supply pow-
ers up prior to the input supply, the enable signal
(SHDN going from low to high) must be delayed until
the input voltage is present to ensure startup.
Current Limit
The MAX1515 provides peak current limiting to protect
the MOSFETs during source/sink overload and short
circuit. During source mode the controller switches the
high-side MOSFET off when the inductor current
exceeds 4.2A. Use the following equation to calculate
the maximum source current:
I_{SOURCE _ MAX} = I_{LIMIT _ P}
−
V_OUT × t_OFF
2×L
where I_{SOURCE_MAX} is the maximum source current,
I_{LIMIT_P} is the source inductor current limit (4.2A typ),
and t_OFF is the fixed off-time. For typical operating con-
ditions and component selection, this results in a maxi-
mum source current of 3.7A.
In sink mode, the MAX1515 does not issue an off-time
until the inductor current is above -3.2A. Use the follow-
ing equation to calculate the maximum sink current:
( ) _{ISINK _ MAX = ILIMIT _ N}
+
V_OUTt_OFF −
2 V_IN −
2L
V_OUT
t_DLY
where I_{SINK_MAX} is the maximum sink current, I_{LIMIT_N}
is the sink inductor current limit (-3.0A typ), t_DLY is the
current-limit propagation delay of approximately 500ns,
and t_OFF is the fixed off-time. For typical operating con-
ditions and component selection, this results in a maxi-
mum sink current of -2.5A.
Soft-Start Current Limit
Soft-start allows a gradual increase of the internal cur-
rent limit to reduce input surge currents at startup and at
exit from shutdown. A timing capacitor, C_SS, placed
from SS to GND sets the rate at which the internal cur-
rent limit is changed. Upon power-up, when the device
comes out of undervoltage lockout (2.6V typ) or after the
SHDN pin is pulled high, a 5µA (typ) constant-current
source charges the soft-start capacitor and the voltage
on SS increases. When the voltage on SS is less than
approximately 0.7V, the current limit is set to zero. As
the voltage increases from 0.7V to approximately 1.8V,
the current limit is adjusted from 0 to the current-limit
threshold (see the Electrical Characteristics). The volt-
age across the soft-start capacitor changes with time
according to the equation:
V_SS
=
I_SS(SRC)
C_SS
×
t
where I_SS(SRC) is the soft-start source current from the
Electrical Characteristics.
The time when full current limit is available is given by:
t = ^{CSS × 1.8V}
I_SS(SRC)
The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:
SSI_LIMIT
=
V_SS − 0.7V
V_REF
×
I_{LIMIT _ P}
where I_{LIMIT_P} is the positive current threshold from the
Electrical Characteristics. The constant-current source
stops charging once the voltage across the soft-start
capacitor reaches 1.8V (Figure 3).
Adjustable Positive Current Limit
The MAX1515 has internal current-limit circuitry that
limits the maximum current through the NMOS to 4.2A.
For applications that require a lower current limit, the
maximum current limit can be reduced by placing a
resistor (R_SS) from SS to GND. The time constant for
the soft-start current limit is R_SS x C_SS.
R_SS
=
⎛
⎝⎜
V_REF ×
I_LIMIT
I_LIMIT
_P
⎞
+ 0.7V_⎠⎟
/ I_SS(SRC)
where I_LIMIT is the desired reduced current limit, and
I_{LIMIT_P} and I_SS(SRC) are taken from the Electrical
Characteristics.
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