SP6121CN-L Datasheet, PDF (13/42 Pages) Sipex Corporation – Low Voltage, Synchronous Step Down PWM Controller

SP6121
160mV
+ ^-+
30µA
-
ISET
ISENSE
RSET
PDRV
VIN
CIN
Q1
L1
COUT
VOUT
NDRV
Q2
Figure 6. Current Limit Setting
The value of R_SET can be properly chosen based on
the desired current limit point I_MAX and the internal
30µA pull down current available on the I_SET pin
according to the following expression:
R_SET
=
I_MAXR_DS(ON) -
I_SET
160mV
where,
I_SET = 30µA (typ) sink current from the I_SET pin.
Kelvin-Sense connections should be made di-
rectly at the drain and source of Q1.
The R_DS(ON) sensing scheme implemented in the
SP6121 provides two additional features that
enhance the performance of the overcurrent
function. First, an internal sample and hold filter
connected after the main current-sense com-
parator, prevents that noise spikes or very short
and mild overload conditions, that could occur
during a load transient, spuriously activate the
current limit circuitry. This typically eliminates
the need of using any external filtering that would
be otherwise required. Additionally, since the
R_DS(ON) has a positive temperature coefficient,
V_OUT > 1.25V
APPLICATIONS INFORMATION
both the 30µA sink current present at the I_SET pin
and the 160mV built-in current limit threshold
have been designed with a positive temperature
coefficient of about 0.33%/C to provide first order
correction for current limit versus temperature.
This compensation relies on the high amount of
thermal coupling that typically exists between the
high side switch Q1 and the SP6121 due to the
compact size of the power supply. With this first
order compensation, the current limit trip point
does not need to be set to an increased level at room
temperature to guarantee a desired output current
level at higher temperatures.
Output Voltage Program
As shown in Figure 7(A), the voltage divider
connecting to the V_FB pin programs the output
voltage according to
( ) ^VOUT
=
1.25
1
+ ^R1
R2
where 1.25V is the internal reference voltage.
Select R2 in the range of 10k to 100k, and R1 can
be calculated using
R1
=
R2(V_OUT -
1.25
1.25)
For output voltage less than 1.25V, a simple
circuit shown in Figure 7(B) can be used in
which V_REF is an external voltage reference
greater than 1.25V. For simplicity, use the same
resistor value for R1 and R2, then R3 is deter-
mined as follows,
R3
=
(V_REF - 1.25)R1
2.5V - V_OUT
V_REF
V_OUT < 1.25V
®
R1
SP6121 ^VFB
R2
R3
®
R1
SP6121
V_FB
R2
A
B
Figure 7(A) A voltage divider connected to the V_FB pin programs the output voltage. (B) A simple circuit using one
external voltage reference programs the output voltages less than 1.25V.
Date: 11/29/04
SP6121 Low Voltage, Synchronous Step Down PWM Controller
13
© Copyright 2004 Sipex Corporation